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>

README.md

@@ -9,7 +9,7 @@ timeseries database. Both client libraries target the schema defined here:
 Neither project should keep its own copy of the DDL. When the schema changes,
 it changes **here**, and both projects re-provision / re-test against it.
 
-Current schema version: **1.3.0** (see [`VERSION`](VERSION); recorded at runtime
+Current schema version: **1.5.0** (see [`VERSION`](VERSION); recorded at runtime
 in the `meta.schema_version` table).
 
 ---
@@ -23,7 +23,7 @@ TSAPI_SERIESDB_SCHEMA/
 ├── db_config.json.example       # copy to db_config.json and edit (gitignored)
 ├── README.md
 └── sql/
-    ├── 00_extensions.sql        # pg_trgm
+    ├── 00_extensions.sql        # pg_trgm, citext
     ├── 01_schemas.sql           # meta / series / log schemas
     ├── 02_sequences.sql         # id sequences
     ├── 03_tables.sql            # meta.* and log.* tables
@@ -78,6 +78,15 @@ database.
 - **Reference tables:** `meta.currencies`, `meta.timezones`, `meta.exchanges`,
   `meta.instruments`, `meta.sessions`, `meta.holidays`, `meta.security_types`,
   `meta.schema_version`.
+- **Holiday calendars (1.5.0):** `meta.holiday_calendars` is the registry -- a
+  calendar exists when it has a row there. It records the coverage range
+  (`covered_from`/`covered_to`: the closure list is complete over it),
+  `description` and `source`. Calendar names are `citext` in every table:
+  stored as given, unique and matched case-insensitively. Foreign keys from
+  `meta.holidays` (cascade on delete), `meta.exchanges` and `meta.instruments`
+  (a calendar still in use cannot be dropped). Clients pass names as untyped
+  parameters (never `::text`) and keep `public` on the `search_path`, so
+  comparisons use the case-insensitive `citext` operators.
 - **Procedures:** `meta.drop_series` / `_if_exists` / `drop_all_series`,
   `meta.update_series_last_timestamp`, `meta.refresh_series_timestamps`,
   `meta.read_series_batch[_cursor|_ohlc]`, `log.notify_event_log`,