feat(timeline): add V77 migration for timeline_events table

Creates timeline_events plus the timeline_event_persons and
timeline_event_documents join tables, all FK columns ON DELETE CASCADE
(a person/document delete drops the join row, the event survives —
V71-class hardening). Two CHECK constraints push integrity to Postgres:
chk_timeline_event_range enforces event_date_end non-null IFF RANGE (a
strict biconditional, intentionally tighter than Document's open-ended
ranges), and chk_timeline_event_precision forbids exactly UNKNOWN while
keeping SEASON/APPROX legal. FK and query-column indexes added up-front
to avoid the V62 retrofit debt. Forward-only, additive DDL.

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

backend/src/main/resources/db/migration/V77__add_timeline_events.sql

@@ -0,0 +1,65 @@
+-- V77: timeline domain foundation (Zeitstrahl) — curated timeline events.
+-- Forward-only, additive DDL. No rollback script: rollback requires manual DROP TABLE
+-- (timeline_event_documents, timeline_event_persons, then timeline_events). See ADR-040.
+--
+-- The date block (event_date / date_precision / event_date_end) mirrors documents' so events
+-- and letters share one rendering path. Two divergences from documents are INTENTIONAL and
+-- enforced here in Postgres (ADR-040):
+--   1. The RANGE rule is a strict biconditional (event_date_end non-null IFF RANGE), unlike
+--      documents' open-ended ranges — a curated event always has a known, closed end.
+--   2. date_precision <> 'UNKNOWN' — only OCR-inferred letters are ever undated; a curated
+--      event always has at least a year. SEASON and APPROX stay legal (Sommer/ca. 1914).
+
+CREATE TABLE timeline_events (
+    id             UUID         NOT NULL DEFAULT gen_random_uuid(),
+    title          VARCHAR(255) NOT NULL,
+    type           VARCHAR(16)  NOT NULL,
+    event_date     DATE         NOT NULL,
+    date_precision VARCHAR(16)  NOT NULL DEFAULT 'YEAR',
+    event_date_end DATE,
+    description    TEXT,
+    created_by     UUID         NOT NULL,
+    created_at     TIMESTAMP,
+    updated_by     UUID         NOT NULL,
+    updated_at     TIMESTAMP,
+    version        BIGINT,
+    CONSTRAINT pk_timeline_events PRIMARY KEY (id),
+    -- event_date_end is non-null IFF precision is RANGE (both directions).
+    CONSTRAINT chk_timeline_event_range
+        CHECK ((date_precision = 'RANGE') = (event_date_end IS NOT NULL)),
+    -- Curated events are never undated. Forbids exactly UNKNOWN — every other
+    -- DatePrecision value (DAY, MONTH, SEASON, YEAR, RANGE, APPROX) stays legal.
+    CONSTRAINT chk_timeline_event_precision
+        CHECK (date_precision <> 'UNKNOWN')
+);
+
+-- Join table: events ↔ persons involved.
+CREATE TABLE timeline_event_persons (
+    timeline_event_id UUID NOT NULL,
+    person_id         UUID NOT NULL,
+    CONSTRAINT pk_timeline_event_persons PRIMARY KEY (timeline_event_id, person_id),
+    CONSTRAINT fk_tep_event
+        FOREIGN KEY (timeline_event_id) REFERENCES timeline_events(id) ON DELETE CASCADE,
+    CONSTRAINT fk_tep_person
+        FOREIGN KEY (person_id) REFERENCES persons(id) ON DELETE CASCADE
+);
+
+-- Join table: events ↔ supporting letters.
+CREATE TABLE timeline_event_documents (
+    timeline_event_id UUID NOT NULL,
+    document_id       UUID NOT NULL,
+    CONSTRAINT pk_timeline_event_documents PRIMARY KEY (timeline_event_id, document_id),
+    CONSTRAINT fk_ted_event
+        FOREIGN KEY (timeline_event_id) REFERENCES timeline_events(id) ON DELETE CASCADE,
+    CONSTRAINT fk_ted_document
+        FOREIGN KEY (document_id) REFERENCES documents(id) ON DELETE CASCADE
+);
+
+-- Indexes added up-front (avoid the V62 FK-index retrofit debt): the two query columns plus
+-- explicit indexes on all four FK columns.
+CREATE INDEX idx_timeline_events_event_date ON timeline_events (event_date);
+CREATE INDEX idx_timeline_events_type ON timeline_events (type);
+CREATE INDEX idx_timeline_event_persons_person_id ON timeline_event_persons (person_id);
+CREATE INDEX idx_timeline_event_persons_event_id ON timeline_event_persons (timeline_event_id);
+CREATE INDEX idx_timeline_event_documents_document_id ON timeline_event_documents (document_id);
+CREATE INDEX idx_timeline_event_documents_event_id ON timeline_event_documents (timeline_event_id);