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feat: config reloading with fsnotify, poller fallback, and signals (#2161)

**Summary**
This PR introduces additional configuration reloading options.
Previously, Auth only supported fsnotify for watching `.env` changes,
which fails on some filesystems. This implementation adds a poller
fallback mode and a signal-triggered reloads.

**Config reloader**

* Multi-source triggers:
  * **fsnotify** for `.env` changes where supported
* **poller fallback** that diffs directory contents for filesystems
without notification support
* **POSIX signals** (default `SIGUSR1`) for operator initiated reloads
(i.e. `systemctl reoad`)
* Debounce via `GracePeriodInterval` to coalesce bursts of changes into
a single reload.
* Invalid configs are logged but ignored, last good configuration
continues to serve.

**Wiring & observability**

* Update `serve_cmd` to construct `NewReloader` from
`ReloadingConfiguration` and log enabled modes/intervals.
* Graceful shutdown handled independently of reloads.
* Errors from watchers and pollers logged clearly, fallbacks engaged
automatically when possible.

**Configuration knobs** (`GOTRUE_RELOADING_*`)

* `NotifyEnabled` (default true)
* `PollerEnabled` (default true), `PollerInterval` (default 10s)
* `SignalEnabled` (default false), `SignalNumber` (default 10 / SIGUSR1)
* `GracePeriodInterval` (default 5s)

**Tests**

* End-to-end reloader tests: fsnotify happy path, poller fallback,
signal path, debounce, invalid configs.
* Poller unit tests for detection semantics and concurrency guard.

---------

Co-authored-by: Chris Stockton <chris.stockton@supabase.io>

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