fix: replace datetime.utcnow() with utcnow()/utcnow_iso() across all source files

Migrates 15 source files from the deprecated datetime.utcnow() API.
Uses utcnow() for in-memory math and utcnow_iso() (strftime format)
for SQLite TEXT column writes to preserve lexicographic sort order.
Also fixes datetime.utcfromtimestamp() in seo/_bing.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/padelnomics/credits.py

@@ -5,9 +5,7 @@ All balance mutations go through this module to keep credit_ledger (source of tr
 and suppliers.credit_balance (denormalized cache) in sync within a single transaction.
 """
 
-from datetime import datetime
-
-from .core import execute, fetch_all, fetch_one, transaction
+from .core import execute, fetch_all, fetch_one, transaction, utcnow_iso
 
 # Credit cost per heat tier
 HEAT_CREDIT_COSTS = {"hot": 35, "warm": 20, "cool": 8}
@@ -44,7 +42,7 @@ async def add_credits(
     note: str = None,
 ) -> int:
     """Add credits to a supplier. Returns new balance."""
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
     async with transaction() as db:
         row = await db.execute_fetchall(
             "SELECT credit_balance FROM suppliers WHERE id = ?", (supplier_id,)
@@ -73,7 +71,7 @@ async def spend_credits(
     note: str = None,
 ) -> int:
     """Spend credits from a supplier. Returns new balance. Raises InsufficientCredits."""
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
     async with transaction() as db:
         row = await db.execute_fetchall(
             "SELECT credit_balance FROM suppliers WHERE id = ?", (supplier_id,)
@@ -116,7 +114,7 @@ async def unlock_lead(supplier_id: int, lead_id: int) -> dict:
         raise ValueError("Lead not found")
 
     cost = lead["credit_cost"] or compute_credit_cost(lead)
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
 
     async with transaction() as db:
         # Check balance
@@ -180,7 +178,7 @@ async def monthly_credit_refill(supplier_id: int) -> int:
     if not row or not row["monthly_credits"]:
         return 0
 
-    now = datetime.utcnow().isoformat()
+    now = utcnow_iso()
     new_balance = await add_credits(
         supplier_id,
         row["monthly_credits"],