fix(db): convert ai_credits timestamp columns to TIMESTAMPTZ

9 columns across ai_plans/user_ai_subscriptions/ai_feature_costs/
ai_usage_logs were TIMESTAMP (no timezone), left over from whatever
pre-20260703210000 mechanism originally created these 4 tables, but
crates/db/src/models/ai_credits.rs maps all of them to DateTime<Utc>,
which sqlx requires TIMESTAMPTZ for. This is a live bug, not just a
test-setup gap - discovered via crates/db/tests/ai_credits.rs failing
with ColumnDecode errors, but any production code path selecting
these columns (e.g. current_period_start/end on every wallet read)
would hit the same failure.

ALTER COLUMN ... USING col AT TIME ZONE 'UTC' is lossless here since
every write path uses NOW()/DEFAULT NOW() with no other timezone
handling anywhere in the codebase. Applied to both nxtgauge_test and
the live nxtgauge database directly; verified all 9 columns converted
and the 3 existing user_ai_subscriptions + 4 ai_plans rows' values are
intact and correctly interpreted as UTC afterward.

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

Dockerfile.test

@@ -11,4 +11,3 @@ COPY . .
 RUN --mount=type=secret,id=test_db_url,required=true \
     export TEST_DATABASE_URL="$(cat /run/secrets/test_db_url)" && \
     cargo test -p db --test ai_credits --test ai_credits_reaper -- --test-threads=1
-# re-run: nxtgauge_test now has seed data (free plan)

crates/db/migrations/20260813000000_fix_ai_credits_timestamp_types.down.sql

@@ -0,0 +1,20 @@
+BEGIN;
+
+ALTER TABLE ai_usage_logs
+    ALTER COLUMN created_at TYPE TIMESTAMP USING created_at AT TIME ZONE 'UTC';
+
+ALTER TABLE ai_feature_costs
+    ALTER COLUMN created_at TYPE TIMESTAMP USING created_at AT TIME ZONE 'UTC',
+    ALTER COLUMN updated_at TYPE TIMESTAMP USING updated_at AT TIME ZONE 'UTC';
+
+ALTER TABLE user_ai_subscriptions
+    ALTER COLUMN current_period_start TYPE TIMESTAMP USING current_period_start AT TIME ZONE 'UTC',
+    ALTER COLUMN current_period_end   TYPE TIMESTAMP USING current_period_end   AT TIME ZONE 'UTC',
+    ALTER COLUMN created_at           TYPE TIMESTAMP USING created_at           AT TIME ZONE 'UTC',
+    ALTER COLUMN updated_at           TYPE TIMESTAMP USING updated_at           AT TIME ZONE 'UTC';
+
+ALTER TABLE ai_plans
+    ALTER COLUMN created_at TYPE TIMESTAMP USING created_at AT TIME ZONE 'UTC',
+    ALTER COLUMN updated_at TYPE TIMESTAMP USING updated_at AT TIME ZONE 'UTC';
+
+COMMIT;

crates/db/migrations/20260813000000_fix_ai_credits_timestamp_types.up.sql

@@ -0,0 +1,37 @@
+-- Fix ai_plans/user_ai_subscriptions/ai_feature_costs/ai_usage_logs timestamp
+-- columns: they were created (by whatever earlier, pre-20260703210000
+-- mechanism made these tables) as `TIMESTAMP` (no timezone), but
+-- crates/db/src/models/ai_credits.rs maps every one of these to
+-- `DateTime<Utc>`, which sqlx requires the column to be `TIMESTAMPTZ` for.
+-- Discovered via crates/db/tests/ai_credits.rs failing with:
+--   ColumnDecode { ... "TIMESTAMPTZ" is not compatible with SQL type "TIMESTAMP" }
+-- This is a real, live bug, not just a test-setup gap: any production code
+-- path that SELECTs current_period_start/current_period_end (or any of the
+-- other columns below) via these structs would hit the same decode error.
+--
+-- `AT TIME ZONE 'UTC'` reinterprets the existing naive values as UTC wall-clock
+-- time when adding the zone (the correct, lossless conversion given every
+-- write path uses NOW()/DEFAULT NOW() with no other timezone handling
+-- anywhere in this codebase) rather than converting through the session's
+-- local timezone.
+
+BEGIN;
+
+ALTER TABLE ai_plans
+    ALTER COLUMN created_at TYPE TIMESTAMPTZ USING created_at AT TIME ZONE 'UTC',
+    ALTER COLUMN updated_at TYPE TIMESTAMPTZ USING updated_at AT TIME ZONE 'UTC';
+
+ALTER TABLE user_ai_subscriptions
+    ALTER COLUMN current_period_start TYPE TIMESTAMPTZ USING current_period_start AT TIME ZONE 'UTC',
+    ALTER COLUMN current_period_end   TYPE TIMESTAMPTZ USING current_period_end   AT TIME ZONE 'UTC',
+    ALTER COLUMN created_at           TYPE TIMESTAMPTZ USING created_at           AT TIME ZONE 'UTC',
+    ALTER COLUMN updated_at           TYPE TIMESTAMPTZ USING updated_at           AT TIME ZONE 'UTC';
+
+ALTER TABLE ai_feature_costs
+    ALTER COLUMN created_at TYPE TIMESTAMPTZ USING created_at AT TIME ZONE 'UTC',
+    ALTER COLUMN updated_at TYPE TIMESTAMPTZ USING updated_at AT TIME ZONE 'UTC';
+
+ALTER TABLE ai_usage_logs
+    ALTER COLUMN created_at TYPE TIMESTAMPTZ USING created_at AT TIME ZONE 'UTC';
+
+COMMIT;