Connection budget: pages, startup and sweeps only touch the databases they need

Adds tests/connection_budget.py, a helper that records which databases a
block of code opens, reads, writes or ATTACHes, and tests/test_connection_budget.py
which asserts budgets for ~60 endpoints, startup, sweeps and telemetry with
50 attached databases.

Fixes for every all-database loop it found:

- Index page: tables, views, hidden tables, columns, primary keys, FTS
  tables and foreign-key relationship counts come from the _internal
  catalog (a few queries for any number of databases). Row counts are only
  run when at most 20 databases need counting (COUNT_MAX_DATABASES);
  above that the page shows table counts plus counts already known.
- Derived-table permissions: allowed_resources() no longer queries every
  database per page; dependency maps are computed by the SchemaWatcher from
  the rows it scans, cached per database, and read back from the catalog
  for databases restored from a persisted catalog.
- catalog_tables gains a type column (PRAGMA table_list: table, virtual,
  shadow) so hidden shadow tables can be identified without a connection.
- PaginatedResources.all() fetches pages of 1000 after the first.
- /-/api and /-/debug/autocomplete use the catalog; /-/schema does one
  permission query and skips databases with nothing visible.
- /-/versions computes SQLite details once per instance, without crossdb
  ATTACHes; detect_json1() caches its answer.
- /-/databases only stats/hashes databases the actor can see.
- Immutable table counts are computed when first needed, not at startup.
- check_databases() (serve, --get) reuses the catalog scan instead of
  opening every database a second time, and reports "file is not a
  database" as a usage error.
- SchemaWatcher: failed scans are retried when the file changes, not on
  every sweep; scan connections skip crossdb ATTACH; catalog_generation
  counter for caches.
- The SQL write UI no longer forces a stat sweep of every database.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwkLo2rJWidpkDVT3HkvNv

docs/internals.rst

@@ -2876,7 +2876,7 @@ The internal database schema is as follows:
         database_name TEXT,
         table_name TEXT,
         rootpage INTEGER,
-        sql TEXT,
+        sql TEXT, type TEXT,
         PRIMARY KEY (database_name, table_name),
         FOREIGN KEY (database_name) REFERENCES catalog_databases(database_name)
     );