fix(auth): exact-match authorization; guard reviews; centralize name validation

Auth + input-validation cluster (CODE-REVIEW-2026-06 #5, #6, #16, #17
+ libra-#36, libra-#51, libra-#52):

- can_access_user_data compares full user ids only. The 8-char prefix
  comparison was a 32-bit space: any prefix collision (or a crafted
  short target id) let one user read another's data.
- can_access_account matches the User-{short} SEGMENT exactly; the
  substring test also matched accounts merely containing it
  (Expenses:Misc-User-deadbeef).
- Manual-payment approve/reject are status-guarded
  (UPDATE ... WHERE status='pending' + rowcount): concurrent admins
  can't double-book. The approve endpoint claims the request BEFORE
  writing the ledger entry and reverts the claim if the write fails,
  so at most one journal entry can exist per request.
- Account-name validation centralized into
  account_utils.validate_account_name (libra-#51) — called from
  crud.create_account (the choke point for every creation path,
  virtual parents allowed a bare root), the admin add-account
  endpoint, and fava_client.add_account at the writer boundary
  (libra-#52).
- crud.create_account translates backend unique-violations into
  AccountExistsError instead of leaking sqlalchemy internals
  (libra-#36); POST /accounts returns 409 on duplicates and 400 on
  malformed names. get_or_create_user_account catches the domain
  error instead of string-matching the SQLite message (which never
  matched on Postgres).

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

fava_client.py

@@ -1775,6 +1775,13 @@ class FavaClient:
         """
         from datetime import date as date_type
 
+        # Defense in depth at the writer boundary (libra-#52): the name is
+        # written verbatim into ledger source below, so validate it HERE,
+        # not only in the endpoints that happen to call this today.
+        from .account_utils import validate_account_name
+
+        validate_account_name(account_name, allow_root_only=True)
+
         if opening_date is None:
             opening_date = date_type.today()