feat: tier modules, GTQ float pricing, and per-subscription billing cycle

[pel19072]

Summary

• tier.price changed from Integer (USD cents) to Float (GTQ). Migration divides existing values by 100.
• tier.price_yearly added (Float, nullable) — annual pricing option in GTQ.
• tier.modules added (JSON, nullable) — controls which sidebar groups a company can access (e.g. ["core", "admin", "management", "automations"]).
• subscription.billing_cycle added (String, NOT NULL, server_default='MONTHLY') — per-company billing period choice (MONTHLY or YEARLY), independent from the tier's default.
• TierPublic schema added for the unauthenticated public pricing page endpoint.
• check_tier_limit() utility added in database_utils/utils/tier_limits.py for hard enforcement of plan usage limits (max_clients, max_products, max_users).
• Updated all affected Pydantic schemas: TierBase, TierUpdate, TierOut, SubscriptionOut, SubscriptionUpdate, SubscriptionWithTier.

Migration notes

• tier.price type change uses postgresql_using='price / 100.0' to convert existing integer cent values to GTQ floats.
• subscription.billing_cycle uses server_default='MONTHLY' so existing rows are populated at the DB level.
• All other changes are additive (new nullable columns).

Test plan

• GitHub Actions migration completes successfully on dev DB
• tier.price existing rows are converted (not truncated to 0)
• New columns (tier.modules, tier.price_yearly, subscription.billing_cycle) exist in the dev DB
• TierPublic schema serializes correctly from a Tier ORM object
• check_tier_limit() raises 402 when count >= limit, passes when limit is None or -1

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