refactor(db): move usage store ownership

[TheSentinel454]

Current reconstructed head

Exact base: codex/issue-2-archived-identities-store at c9841016d226d8c75463862d3617999cd3af1ec3
Exact head: codex/issue-2-usage-store at fad0f34b0d5abb9b24ac2c125e39654fec44c124

This current head removes crates/buzz-db/tests/store_ownership.rs; no replacement path-sensitive ownership test is introduced. Apart from removing that complete test-file diff, the production patch is byte-for-byte identical to the previously reviewed slice. This remains part of tracker #2 and the #17/#19 acceptance work.

Independent exact-head review from a separate clean Blox workstation found no issues. Current-head evidence passed formatting, strict buzz-db clippy, 111 non-PostgreSQL library tests with 200 PostgreSQL tests ignored, the observability source test, relay consumer compilation, exact ownership/unique-span review checks, and 9 usage PostgreSQL tests on native PostgreSQL where applicable.

Summary

Continue tracker #2 by making usage.rs own usage rollups and the usage-poller advisory-lock lifecycle. The rollup records, SQL, and focused PostgreSQL tests already lived there; this child moves the ten rollup Db methods, UsageMetricsLeader, try_lock_usage_metrics, its focused scratch-database test, and all eleven datastore spans out of lib.rs.

The existing crate-root UsageMetricsLeader API is preserved with a re-export. This also advances #17 and #19.

Stack

• Tracker: TheSentinel454/buzz#2
• Test ownership acceptance: #17
• Span ownership acceptance: #19
• Exact base: codex/issue-2-archived-identities-store at 8d3042d (#6811)
• Exact head: codex/issue-2-usage-store at 27378c8

Domain moved

• Ten usage rollup facades: community, user, channel, message, relay-member, workflow, Git-repository, active-user, active-channel, and community-host queries
• Detached-session advisory-lock acquisition and UsageMetricsLeader
• The advisory-lock single-owner/release test, now beside the usage poller store boundary; its local scratch-database setup preserves isolation from any live poller
• All eleven existing fixed-name PostgreSQL datastore spans

crates/buzz-db/src/lib.rs falls from 3,938 to 3,772 lines.

Non-goals

• Prometheus label emission or relay poll scheduling
• Usage SQL, interval literals, classification, advisory-lock key, detached-connection lifetime, timeouts, schema, or client-visible behavior changes
• Generic store traits, raw pool access, executor migration, or crate reorganization

Risk

Moderate review surface and low semantic risk. Production methods and spans moved intact. The detached physical connection remains acquired through the PR #6700 observability wrapper and retains the same five-second liveness timeout and drop-to-unlock behavior. Focused test setup is now module-local but creates, migrates, and drops the same isolated scratch database.

Blox verification

Author workstation: buzz-tornquist-issue-2-store-stack (2046520), exact head de6216580a94d060390ef8f319bad5ea7f242670.

• cargo fmt --all --check
• git diff --check 33129b56f3cc319ce98f9400ef90412a558490e2..HEAD
• cargo clippy -p buzz-db --all-targets -- -D warnings
• cargo clippy -p buzz-relay --all-targets -- -D warnings
• cargo test -p buzz-db --lib: 108 passed, 200 ignored
• cargo test -p buzz-db --test observability_source: 1 passed
• Native PostgreSQL 17.11, migrations 1–32 already applied: all 9 usage tests passed, including detached-session advisory-lock ownership/release in an isolated scratch database
• cargo test -p buzz-relay --lib: 909 passed, 48 ignored
• Commit hooks passed without bypass; the disposable workstation had no user signing key, so the commit itself is unsigned

Independent exact-head Blox review: no findings. Fresh workstation buzz-tornquist-pr-6812-review (2055068) verified exact geometry and reproduced format/diff checks, DB and relay clippy, DB library (108 passed, 200 ignored), ownership (19), observability (1), all 9 native-PostgreSQL usage tests, and relay library (909 passed, 48 ignored). Review artifact SHA-256: 0927b1f96c52254622403b52cdf302fe9733e3c9683fe891e2619da88641bca0.

Remaining tracker work

Next: admin moderation projections, partition/event backfill maintenance, deletion-store facades, the remaining cross-domain helper audit, and the final runtime-boundary/test assessment.

Superseded pre-comment restack verification

PR #6700 merged before publication completed. This layer was restacked onto current main through the exact parent named above; the final cumulative tip is 2ddcc8a. Cumulative author gates passed: formatting and diff checks; buzz-db and buzz-relay all-target clippy with -D warnings; DB lib 111 passed / 200 ignored; ownership 22/22; observability 1/1; the full isolated PostgreSQL domain matrix; and relay lib 910 passed / 49 ignored.

Result

No findings.

Exact revision and isolation

• Base: 86bb22d686c88735e505b47710a5de5ceefcce8d
• Head: d21fc19f455726960fff633b491d031697af6694
• Verified head parent and merge-base: exact base above
• Reviewer workstation: buzz-tornquist-pr-6812-final-review (2057669)

The fresh shallow workstation was clean and unclaimed, had no competing commands, and had no Buzz production relay credentials or ownership-report file.

Review conclusions

• UsageMetricsLeader, its bounded liveness check, the detached advisory-lock acquisition, all ten usage-query facades, and the focused lock test move together into usage.rs.
• The crate-root UsageMetricsLeader re-export preserves the consumer path. Advisory-lock SQL, observed writer acquisition, physical-connection detachment, timeout, and drop-release semantics are unchanged.
• Query SQL and public result types were already domain-owned and remain unchanged. Eleven moved logical operations retain one span apiece.

Verification

• Diff check and Rust formatting: passed.
• Buzz DB and relay all-target clippy with -D warnings: passed.
• Buzz DB library: 111 passed, 200 ignored.
• Native PostgreSQL 17.11, migrations complete: all nine usage tests passed, including single-owner advisory lock and release-on-drop.
• Final detached head remained clean; no duplicate datastore-span names were found.

Artifacts: pr-6812-final-review-artifacts.tgz, SHA-256 244c3cc095319a622bfd204c5c7bc472f4a0ad403cbe97d5620b4f852625bcd6. Archive and internal checksums were verified locally.

Comment-addressed restack

Review follow-up on #6777 removed only the low-value replaceable ownership source test. This PR was restacked onto its rewritten parent; its production patch is unchanged.

• Exact base: 8d3042d25ff766b1e90979a016eb60aa04021562
• Exact head: 27378c8f13143efed9815abc17e4c7efba572d2f
• Final cumulative tip: 6fa2f104d42c6ba85bdf62e7ccb74ceaf4a84f67
• Per-layer patch-ID and tree audits confirm this PR’s production diff is unchanged from its pre-comment head.
• Cumulative Blox gate: formatting and diff checks; strict buzz-db/buzz-relay Clippy; DB lib 111 passed / 200 ignored; ownership 21/21; observability 1/1; every moved PostgreSQL test; relay lib 910 passed / 49 ignored.
• Independent re-review at this exact head: no findings; fresh exact-parent/head Blox review passed fmt/diff, strict buzz-db/buzz-relay Clippy, DB lib and current ownership/observability/unique-span guards, 9 usage PostgreSQL tests, and relay compilation.