Consistency sweep: enum members, keyword status codes, SimpleCookie

analytics.py filters by TransactionType.COMPRA instead of the string
literal; budget.py uses keyword HTTPException args; the agent
middleware parses cookies with stdlib SimpleCookie instead of a regex
(ARCH-10); auth.py's lazy imports are hoisted — there was never an
actual auth/db cycle (ARCH-20); transactions.py hoists or_/and_.
Verified: dev container boots, agent auth paths return 401 for
missing/garbage cookies.

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

backend/app/api/v1/endpoints/budget.py

@@ -248,9 +248,9 @@ def upsert_balance_override(
     _user: str = Depends(get_current_user),
 ):
     if year < MIN_YEAR or year > MAX_YEAR:
-        raise HTTPException(400, f"Year must be between {MIN_YEAR} and {MAX_YEAR}")
+        raise HTTPException(status_code=400, detail=f"Year must be between {MIN_YEAR} and {MAX_YEAR}")
     if year == FRESH_START_YEAR and month < FRESH_START_MONTH:
-        raise HTTPException(400, f"Cannot override before {FRESH_START_YEAR}-{FRESH_START_MONTH:02d}")
+        raise HTTPException(status_code=400, detail=f"Cannot override before {FRESH_START_YEAR}-{FRESH_START_MONTH:02d}")
 
     existing = session.exec(
         select(BalanceOverride).where(
@@ -288,6 +288,6 @@ def delete_balance_override(
         )
     ).first()
     if not existing:
-        raise HTTPException(404, "No override found for this month")
+        raise HTTPException(status_code=404, detail="No override found for this month")
     session.delete(existing)
     session.commit()