chore: eliminate all deprecation warnings from tests and linter

Warnings fixed:
- datetime.utcnow() → datetime.now(UTC) across 48+ files (src/ + tests/)
- datetime.utcnow (callback ref) → lambda: datetime.now(UTC) in model fields (18 files)
- Pydantic class Config → model_config = ConfigDict(...) (16 files)
- Pydantic .dict() → .model_dump() (8 files)
- ConfigDict(allow_population_by_field_name=True) → validate_by_name=True
- SQLAlchemy declarative_base() import path updated
- FastAPI on_event → lifespan context manager (app.py)
- Import sorting (ruff I001) auto-fixed across all files
- Fixed broken re-export chains that ruff F401 cleanup broke:
  _validate_bcp47: service.py now imports from dictionary_validation directly
  job_to_response: _job_routes.py and test imports from service_utils directly
  fetch_datasource_metadata: restored re-export in service.py
- Added missing TranslateJobService import in _job_routes.py (was deleted by F401)
- Added ConfigDict(protected_namespaces=()) for DashboardDatasetItem schema field
- pytest.ini: replaced deprecated importmode with asyncio_mode

All 440 tests pass with zero deprecation warnings.

backend/src/services/sql_table_extractor.py

@@ -8,12 +8,12 @@
 # @INVARIANT Only exact schema.table matches (case-insensitive); unqualified references are NOT matched.
 # @INVARIANT Returns a set[str] of fully-qualified table names (lowercased for case-insensitive matching).
 
+from collections.abc import Iterable
 import re
-from typing import Iterable
 
 import sqlparse
-from sqlparse.sql import Identifier, Parenthesis, Token, TokenList
-from sqlparse.tokens import Literal, Name, Punctuation, Whitespace
+from sqlparse.sql import Token, TokenList
+from sqlparse.tokens import Literal
 
 # ── Phase 1: Jinja span detection ───────────────────────────