fix(geoip): reuse one City reader across reloads

[Rushaway]

Problem

!reload produces an error traceback and drops the !ow command (#174):

INFO  | Setup Unloaded 23 commands!
ERROR | Traceback (most recent call last):
  File ".../torchlight/CommandHandler.py", line 52, in Setup
    command = subklass(
  File ".../torchlight/Commands.py", line 425, in __init__
    self.geo_ip = geoip2.database.Reader(f"{self.config_folder}/{self.city_filename}")
  ...
  File ".../maxminddb/__init__.py", line 80, in open_database
    return cast(Reader, _extension.Reader(database, mode))

CommandHandler.Setup() rebuilds every command on each reload, so
OpenWeather.__init__ opened a brand-new geoip2.database.Reader (an mmap of
the ~60 MB City database) every time. #173 added close() on the outgoing
command so that mapping no longer leaked until GC (#55), but re-opening the
database on every !reload is still fragile — and when the open fails, the
exception propagates out of __init__, aborts the command in Setup(), and is
logged as an error. The database is read-only and static; there is no reason to
tear it down and map it again on a config reload.

Fix

• New torchlight.GeoIP module owns the reader. Only Commands is passed to
  importlib.reload, so a cache in its own module survives !reload.
• get_city_reader() opens the database once per path and reuses it. It
  reopens only when the file's mtime changes (e.g. after a GeoIP update), closing
  the previous reader once the new one is open. If a reopen fails, it keeps
  serving the previous reader instead of losing GeoIP entirely.
• OpenWeather no longer raises out of __init__ when the database can't be
  opened: it logs the failure, leaves geo_ip unset, and !ow with no argument
  replies asking for a city instead of crashing. !ow <city> is unaffected.
• OpenWeather no longer needs its close() override; the generic
  BaseCommand.close() hook from fix(#55): close YoutubeDL, geoip reader, and the audio-clip retain graph #173 stays for future use.

Testing

• ruff check ., ruff format . --diff, mypy — clean (the two pre-existing
  MyInstants.py mypy errors are unrelated).
• Standalone test of get_city_reader with a stubbed geoip2.database.Reader
  covering: reuse across calls, surviving repeated Setup() with no file change,
  reopen on mtime change (old reader closed), reopen failure falling back to the
  previous reader, missing file with a warm cache, and first-open failure
  propagating.

Fixes #174

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