build: require Python 3.11 and pandas 3

- requires-python 3.11 and pandas 3.0.6; the CI matrix runs Python 3.11 to 3.14 and ruff targets 3.11
- datetime.UTC in place of timezone.utc, and the rating, action, band, analyst and asset enums are StrEnum, which formats a member as its value

cli/models.py

@@ -1,7 +1,7 @@
-from enum import Enum
+from enum import StrEnum
 
 
-class AnalystType(str, Enum):
+class AnalystType(StrEnum):
     MARKET = "market"
     # Wire value stays "social" for saved-config and string-keyed-caller
     # back-compat; the user-facing label is "Sentiment Analyst".
@@ -10,6 +10,6 @@ class AnalystType(str, Enum):
     FUNDAMENTALS = "fundamentals"
 
 
-class AssetType(str, Enum):
+class AssetType(StrEnum):
     STOCK = "stock"
     CRYPTO = "crypto"