fix(db): add PRIMARY KEY to group_permissions; promote tbmp UNIQUE to PK

V63 deduplicates any phantom (group_id, permission) rows accumulated since
the initial schema. V64 sets NOT NULL on permission and adds pk_group_permissions.
V65 renames uq_tbmp_block_person to pk_tbmp for naming-convention consistency.
Integration tests confirm each constraint via pg_catalog.pg_constraint. Closes #469 (partial).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V63__deduplicate_group_permissions.sql

@@ -0,0 +1,7 @@
+-- Remove duplicate (group_id, permission) rows that accumulated without a UNIQUE constraint.
+-- Keeps the row with the smallest ctid (earliest physical insertion order).
+DELETE FROM group_permissions a
+USING group_permissions b
+WHERE a.ctid < b.ctid
+  AND a.group_id = b.group_id
+  AND a.permission = b.permission;

backend/src/main/resources/db/migration/V64__group_permissions_primary_key.sql

@@ -0,0 +1,11 @@
+-- Add NOT NULL and PRIMARY KEY to group_permissions.
+-- Requires V63 to have run first (no duplicates can remain).
+--
+-- After this migration, future seed migrations can use:
+--   INSERT INTO group_permissions ... ON CONFLICT DO NOTHING
+-- instead of the INSERT ... WHERE NOT EXISTS pattern used before V64.
+ALTER TABLE group_permissions
+    ALTER COLUMN permission SET NOT NULL;
+
+ALTER TABLE group_permissions
+    ADD CONSTRAINT pk_group_permissions PRIMARY KEY (group_id, permission);

backend/src/main/resources/db/migration/V65__tbmp_promote_unique_to_pk.sql

@@ -0,0 +1,8 @@
+-- Promote the de-facto unique constraint on transcription_block_mentioned_persons to a named PK.
+-- uq_tbmp_block_person (added in V57) is backed by a B-tree index identical to a PK;
+-- this rename makes the naming convention explicit (pk_* vs uq_*).
+ALTER TABLE transcription_block_mentioned_persons
+    DROP CONSTRAINT uq_tbmp_block_person;
+
+ALTER TABLE transcription_block_mentioned_persons
+    ADD CONSTRAINT pk_tbmp PRIMARY KEY (block_id, person_id);