fix(schemas): use enum types as field types + enforce symbol length

- Replace all Literal[...] type annotations with corresponding enum classes
  (TickerAction, TimeHorizon, MarketOutlook, VideoStatus, TranscriptSource)
  for MeetKevinTickerMention, MeetKevinAnalysis, and API response models
  (VideoSummary, VideoDetail, StockMention, StockSummary, TimelineBucket)
- Add min_length=1, max_length=10 validation to MeetKevinTickerMention.symbol
- Split test_conviction_edge_cases into two separate boundary tests
- Strengthen test_valid_ticker_mention with assertions for all 6 fields
- Trim no-information docstrings from TranscriptSegment, StockTimeline
- All 60 schema tests pass

tests/test_schemas.py

@@ -593,7 +593,7 @@ class TestTokenResponse:
 
 class TestMeetKevinTickerMention:
     def test_valid_ticker_mention(self) -> None:
-        from shared.schemas.meet_kevin import MeetKevinTickerMention
+        from shared.schemas.meet_kevin import MeetKevinTickerMention, TickerAction, TimeHorizon
 
         mention = MeetKevinTickerMention(
             symbol="AAPL",
@@ -604,7 +604,11 @@ class TestMeetKevinTickerMention:
             video_timestamp_seconds=120,
         )
         assert mention.symbol == "AAPL"
+        assert mention.action == TickerAction.BUY
         assert mention.conviction == 0.85
+        assert mention.time_horizon == TimeHorizon.MONTHS
+        assert mention.rationale_quote == "Strong earnings growth expected"
+        assert mention.video_timestamp_seconds == 120
 
     def test_symbol_auto_uppercases(self) -> None:
         from shared.schemas.meet_kevin import MeetKevinTickerMention
@@ -642,28 +646,29 @@ class TestMeetKevinTickerMention:
                 rationale_quote="Negative conviction",
             )
 
-    def test_conviction_edge_cases(self) -> None:
+    def test_conviction_boundary_zero_valid(self) -> None:
         from shared.schemas.meet_kevin import MeetKevinTickerMention
 
-        # Test 0.0
-        m1 = MeetKevinTickerMention(
+        m = MeetKevinTickerMention(
             symbol="GOOG",
             action="avoid",
             conviction=0.0,
             time_horizon="unspecified",
             rationale_quote="No confidence",
         )
-        assert m1.conviction == 0.0
+        assert m.conviction == 0.0
 
-        # Test 1.0
-        m2 = MeetKevinTickerMention(
+    def test_conviction_boundary_one_valid(self) -> None:
+        from shared.schemas.meet_kevin import MeetKevinTickerMention
+
+        m = MeetKevinTickerMention(
             symbol="MSFT",
             action="buy",
             conviction=1.0,
             time_horizon="long_term",
             rationale_quote="Maximum confidence",
         )
-        assert m2.conviction == 1.0
+        assert m.conviction == 1.0
 
     def test_timestamp_optional(self) -> None:
         from shared.schemas.meet_kevin import MeetKevinTickerMention