fix: normalize date-bucket keys for Postgres dashboard charts

Dashboard charts (activity, message-activity, node-count) rendered as
flat zeros on Postgres because func.date() returns a str on SQLite but
a datetime.date on Postgres — the dict lookup by string key always
missed. Fixed with a dialect-neutral _date_bucket_key() helper and
pinned the Postgres session timezone to UTC at the engine level.

Also adds dual-backend test infrastructure (TEST_DATABASE_BACKEND env
var), per-worker Postgres databases for pytest-xdist isolation, and
strengthened regression tests asserting non-zero date buckets.

docs/upgrading.md

@@ -73,6 +73,21 @@ Downtime is required while writers are stopped; the source SQLite file is never
 
 > **Managed Postgres / non-superuser roles:** the migration disables foreign-key triggers during the copy via `session_replication_role = replica`, which requires a superuser. When the target role is not a superuser (typical for managed Postgres), the command automatically falls back to copying in parent-first order instead. Pass `--no-replication-role` to force the fallback explicitly.
 
+### Dashboard Chart Fix (Postgres)
+
+After enabling Postgres, the dashboard charts (activity, message-activity, node-count) may render as flat zeros. This is a known issue caused by a dialect mismatch in the date-bucketing query — `func.date()` returns a `str` on SQLite but a `datetime.date` on Postgres, causing the dict lookup to miss. A fix normalizes the key to a canonical `"%Y-%m-%d"` string and pins the Postgres session timezone to UTC.
+
+The fix takes effect **after one `REDIS_CACHE_TTL_DASHBOARD` period** (default 30 seconds) — stale all-zero cached responses expire automatically. For operators who have configured a substantially longer TTL, either wait one TTL period or flush the three dashboard cache key prefixes:
+
+```bash
+redis-cli -h <redis-host> DEL \
+  "$(echo -n 'hub:dashboard:activity*' | xargs redis-cli KEYS)" \
+  "$(echo -n 'hub:dashboard:message-activity*' | xargs redis-cli KEYS)" \
+  "$(echo -n 'hub:dashboard:node-count*' | xargs redis-cli KEYS)"
+```
+
+No database migration or configuration change is required — the fix is automatic.
+
 ## v0.13.0
 
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