fix(db): ride out a brief database outage mid-request; explain it if it persists

The operator's 500 was NOT the startup gap I attributed it to. They found it by
logging in and getting Steward's own error page -- which means create_app had
completed and the app was serving, so the gaierror came from a request handler
acquiring a connection, not from boot. Neither prior fix covers that: the
startup retry never runs, and pool_pre_ping only helps once the database is
back, since its replacement connect fails too while the container is gone.

Adds a before_request gate that acquires a pooled connection with a short
bounded retry (~1.75s over 4 tries) so a database restart is ridden out
invisibly, and renders a distinct 503 "database unavailable" page when the
budget is exhausted. The request budget is deliberately far shorter than the
startup one: nobody watches a container boot, but somebody is watching this
page load, and a page that hangs is worse than one that says what is wrong.

/health and static are exempt -- a liveness probe must stay answerable while
the database is down, or a dependency outage triggers a restart loop.

Credential and missing-database errors still propagate rather than being
reported as "unavailable", which would send the operator chasing the wrong
problem entirely.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

steward/app.py

@@ -3,9 +3,12 @@ from __future__ import annotations
 import asyncio
 import logging
 from pathlib import Path
-from quart import Quart, render_template
+from quart import Quart, render_template, request
 from .config import load_bootstrap
-from .database import init_db, DB_CONNECT_TIMEOUT_SECONDS
+from .database import (
+    init_db, ensure_database_reachable, DatabaseUnavailable,
+    DB_CONNECT_TIMEOUT_SECONDS,
+)
 
 logger = logging.getLogger(__name__)
 
@@ -245,6 +248,29 @@ def create_app(
     async def health():
         return {"status": "ok"}
 
+    # ── 11b. Database availability gate ────────────────────────────────────────
+    # Every page in Steward reads the database, so a database that has gone away
+    # under a running app turns each request into an opaque 500 (this is how a
+    # login attempt surfaced a bare gaierror). Acquire a connection up front,
+    # retrying briefly to ride out a restart, and answer honestly if it stays
+    # down rather than failing deep inside a handler with a generic error.
+    if not testing:
+        @app.before_request
+        async def _database_gate():
+            # /health is a liveness probe for the container itself — it must
+            # stay answerable while the database is down, or a restart loop
+            # gets triggered by a dependency outage. Static files need no DB.
+            if request.endpoint in ("health", "static"):
+                return None
+            try:
+                await ensure_database_reachable(app._db_engine)
+            except DatabaseUnavailable as exc:
+                logger.error("Database unreachable while serving %s: %s",
+                             request.path, exc)
+                return await render_template(
+                    "errors/database_unavailable.html"), 503
+            return None
+
     # ── 12. Error handlers ─────────────────────────────────────────────────────
     @app.errorhandler(404)
     async def not_found(_):