fix(meet-kevin): API smoke-test bugs from Task 17 QA

Three issues caught during end-to-end manual QA against docker-compose:

1. SAEnum field columns serialized to Python enum NAMES ('DISCOVERED')
   but the DB enum had VALUES ('discovered'). Added `values_callable`
   to all 5 SAEnum() declarations in shared/models/meet_kevin.py so they
   emit values, matching the migration's enum literals.

2. /dashboard's "last 7 days" / "last 14 days" filters used
   `func.cast("7 days", type_=None)` which produced NullType DDL.
   Replaced with `text("now() - interval '7 days'")`.

3. /dashboard's outlook trend query repeated `func.date_trunc("day", col)`
   in SELECT, GROUP BY and ORDER BY — Postgres treats each as a separate
   parameterized expression. Hoisted into a single `day_trunc` variable
   so all three clauses reference the same SQL fragment.

All 11 /api/meet-kevin/* endpoints now return valid JSON against a
docker-compose Postgres seeded with one analyzed video + NVDA mention.

services/api_gateway/routes/meet_kevin.py

@@ -612,14 +612,15 @@ async def get_dashboard(
             }
 
         # Top conviction last 7 days
-        seven_days_ago = func.now() - func.cast("7 days", type_=None)
+        from sqlalchemy import text
+        seven_days_ago = text("now() - interval '7 days'")
         top_conviction_result = await session.execute(
             select(
                 KevinStockMention.symbol,
                 func.max(KevinStockMention.conviction).label("max_conviction"),
                 func.count().label("mention_count"),
             )
-            .where(KevinStockMention.created_at >= func.now() - func.cast("7 days", type_=None))
+            .where(KevinStockMention.created_at >= text("now() - interval '7 days'"))
             .group_by(KevinStockMention.symbol)
             .order_by(desc(func.max(KevinStockMention.conviction)))
             .limit(10)
@@ -634,18 +635,17 @@ async def get_dashboard(
         ]
 
         # 14-day outlook trend: date × direction → count
+        # Use a single trunc expression so SELECT/GROUP BY/ORDER BY reference the same SQL fragment
+        day_trunc = func.date_trunc("day", KevinAnalysis.created_at)
         outlook_result = await session.execute(
             select(
-                func.date_trunc("day", KevinAnalysis.created_at).label("day"),
+                day_trunc.label("day"),
                 KevinAnalysis.market_outlook_direction,
                 func.count().label("count"),
             )
-            .where(KevinAnalysis.created_at >= func.now() - func.cast("14 days", type_=None))
-            .group_by(
-                func.date_trunc("day", KevinAnalysis.created_at),
-                KevinAnalysis.market_outlook_direction,
-            )
-            .order_by(func.date_trunc("day", KevinAnalysis.created_at))
+            .where(KevinAnalysis.created_at >= text("now() - interval '14 days'"))
+            .group_by(day_trunc, KevinAnalysis.market_outlook_direction)
+            .order_by(day_trunc)
         )
         outlook_trend = [
             {