Schema 1.5.0: holiday calendar registry, citext names; drop num_records

For the coverage-aware holiday-calendar API in TSAPI core (db_base
holiday_calendar__get / __set_coverage / __set_metadata / __upsert).

- meta.holiday_calendars (name, description, covered_from, covered_to,
  source, updated_at): a calendar exists when it has a row here, not
  when it has holidays (retires the 9999-12-31 sentinel row).
  covered_from/to: the range over which the closure list is complete;
  both set or both NULL, from <= to (holiday_calendars_coverage_check).
- Calendar names are citext in every table (registry, holidays.calendar_name,
  exchanges.default_holiday_calendar, instruments.holiday_calendar):
  stored as given, unique and matched case-insensitively -- also in the
  foreign keys -- as the native TSAPI backend does with COLLATE NOCASE.
  New extension citext (00_extensions.sql). Name check: non-empty,
  trimmed, <= 50 characters.
- FKs: holidays -> registry ON DELETE CASCADE; exchanges/instruments ->
  registry ON DELETE RESTRICT (a calendar in use cannot be dropped);
  ON UPDATE CASCADE everywhere.
- meta.series.num_records dropped. Only some write paths maintained it
  (never C++), it sometimes held an estimate, and it was NULL for a series
  never written to. Clients ask the table: count(*), or
  SELECT EXISTS (SELECT 1 FROM <table>) for "is it empty?".
- find_schema_problems() covers the new table, columns, constraints and
  extension, and runs with search_path = pg_catalog so format_type()
  reports 'public.citext' whatever the caller's search_path.

Verified on a fresh database (PG 17): run_setup.py installs it and
find_schema_problems() returns zero rows, also for a caller without
public on the search_path; constraint behaviour checked by hand
(case-insensitive duplicate refused, FK matches any spelling, RESTRICT
and CASCADE, coverage and name checks).

Re-provision databases (no migration).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sql/00_extensions.sql

@@ -2,3 +2,7 @@
 -- Generated: 2025-10-15T12:24:56.026074
 
 CREATE EXTENSION IF NOT EXISTS pg_trgm SCHEMA public;
+
+-- Case-insensitive text for holiday calendar names (1.5.0): stored as given,
+-- compared and keyed case-insensitively -- including in foreign keys.
+CREATE EXTENSION IF NOT EXISTS citext SCHEMA public;