Replace Postgres with SQLite

Ten members and a handful of songs a week never needed a database server,
and the server was the last thing making this a two-container deployment.
modernc.org/sqlite is pure Go, so CGO_ENABLED=0 survives and the dependency
count is unchanged: pgx out, sqlite in.

The port stayed small because the driver matches $1-style placeholders
against argument ordinals exactly as pgx does, so no query needed rewriting
for parameters. What did change:

- timestamptz becomes timestamp holding UTC 'YYYY-MM-DD HH:MM:SS'. The
  declared type is what makes the driver return time.Time, and the
  fixed-width UTC string is what makes ordering and comparison against
  datetime('now') mean what they say.
- interval has no equivalent: sessions.idle_ttl is seconds, and the review
  edit window travels as a SQLite date modifier string.
- No stddev_pop, so the divisive and unified boards spell the population
  formula out, guarded with max(0.0, ...) because cancellation returns a
  tiny negative when every score is identical.
- foreign_keys is off by default, so the cascades only exist because the
  pragma is set on every connection.

Drops the postgres service, its healthcheck, the depends_on gate, the
startup retry loop and POSTGRES_PASSWORD. ./storage is now the whole
backup. Tests get a fresh database file per test and run everywhere
instead of skipping without TEST_DATABASE_URL.

docs/decisions.md

@@ -209,3 +209,30 @@ says so.
 
     The coverage assumption that shaped the earlier sketch was wrong and is corrected in `later.md`:
     LRCLIB has synced lyrics for a good share of Finnish rock, not almost none.
+46. **SQLite instead of Postgres — this reverses entries 1 and 3** (2026-08-02). Ten members and a
+    handful of songs a week never needed a database server, and the server was the last thing making
+    this a two-container deployment. `modernc.org/sqlite` is pure Go, so `CGO_ENABLED=0` survives and
+    the dependency count does not change: `pgx` out, `sqlite` in. What it buys: one container, one
+    bind mount that is the entire backup, no `pgdata`, no healthcheck-gated `depends_on`, no startup
+    retry loop, and tests that run anywhere instead of skipping without `TEST_DATABASE_URL`.
+
+    The port was smaller than expected, because the driver matches `$1`-style placeholders against
+    argument ordinals exactly as pgx does — so no query needed rewriting for parameters. What did
+    change:
+    - **`timestamptz` → `timestamp` holding UTC `YYYY-MM-DD HH:MM:SS`.** The declared type is what
+      makes the driver return `time.Time`; the fixed-width UTC string is what makes `order by
+      created_at` and `expires_at > datetime('now')` mean what they say. `_time_format=datetime` and
+      `_timezone=UTC` on the DSN make Go write exactly the shape `datetime('now')` produces, so the
+      two sources of a timestamp are comparable.
+    - **`interval` has no equivalent.** `sessions.idle_ttl` is seconds as an integer, and the review
+      edit window travels as a SQLite date modifier string (`-1800 seconds`).
+    - **No `stddev_pop`.** The divisive and unified boards use the population formula written out,
+      guarded with `max(0.0, …)` because floating-point cancellation returns a tiny negative when
+      every score is identical, and `sqrt` of that is null.
+    - **`foreign_keys` is off by default in SQLite**, so every `on delete cascade` in the schema is
+      decoration without the pragma. It is set on the DSN alongside WAL, `busy_timeout` and
+      `_txlock=immediate`.
+
+    Taken while there was still no data: the tables were recreated rather than converted, same as
+    entry 45. What would reverse this: enough concurrent writers that one writer is a real limit, or
+    wanting the database on a different box from the audio files.