fix: complete user-delete FK lockstep across PG and SQLite schemas

The prior user-deletion work updated the PG schema and a live-PG migration
but left the canonical schema definitions inconsistent, breaking user
deletion on fresh PG installs and on all SQLite dev installs.

- schema.sql: add ON DELETE SET NULL to context_files.updated_by (was the
  only user FK missing it; fresh PG installs could not delete an authoring
  user).
- schema_sqlite.sql: bring five user_id FK columns into lockstep with PG
  (drop NOT NULL, add ON DELETE SET NULL): project_context.updated_by,
  context_files.updated_by, change_requests.submitted_by,
  reviews.reviewer_id, audit_log.user_id.
- schema_sqlite.sql: remove the audit_log append-only UPDATE/DELETE triggers.
  ON DELETE SET NULL on audit_log.user_id is an UPDATE the trigger aborted,
  so deleting any user who had ever logged in failed. This mirrors schema.sql,
  which dropped the equivalent PG triggers in fc1a2f5; append-only is enforced
  at the application layer (db.py only INSERTs into audit_log).
- db.py: user_delete no longer swallows non-FK exceptions on the SQLite path
  (Exception masked sqlite3.IntegrityError); only FK violations map to the
  soft "user_has_references" response, everything else propagates. PG
  rollback-on-any-error (shared-connection cascade fix) is preserved.
- db.py: document that SQLite cannot ALTER FK constraints in place; existing
  dev DBs must be recreated to pick up these changes.
- server.py: the global 409 handler no longer leaks raw psycopg text (index
  names, column expressions) to API callers; it is logged instead.
- migrate_user_fk_set_null.py: use the column from FKS_TO_FIX directly instead
  of re-deriving it from the constraint name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/src/ctxd/schema.sql

@@ -80,7 +80,7 @@ CREATE TABLE context_files (
     file_path    TEXT NOT NULL,
     content      TEXT NOT NULL DEFAULT '',
     version      INTEGER NOT NULL DEFAULT 1,
-    updated_by   TEXT REFERENCES users(user_id),
+    updated_by   TEXT REFERENCES users(user_id) ON DELETE SET NULL,
     updated_at   TEXT NOT NULL DEFAULT to_char(now() at time zone 'utc', 'YYYY-MM-DD"T"HH24:MI:SS"Z"'),
     UNIQUE(project_id, file_path)
 );