store: fix migration 0026's silent-cascade-delete FK toggle bug

Migration 0026 (activities table rebuild for the per-user unique
constraint) issued its own PRAGMA foreign_keys = OFF/ON inside the
migration's SQL content, but the whole migration file runs inside one
db.go tx.Begin()/tx.Exec()/tx.Commit() transaction, and SQLite documents
PRAGMA foreign_keys as a no-op once a transaction is open. As a result FK
enforcement never actually got disabled, so DROP TABLE activities
triggered SQLite's implicit DELETE FROM semantics, firing ON DELETE
CASCADE on every row in laps, activity_samples, and kind_assignments for
every activity -- silently, with no error. On a real upgrade with synced
data this would have permanently destroyed all lap/sample/classification
history. It was masked because every existing migration test runs
migrations back-to-back on an empty temp database with no pre-existing
child rows.

Fix: add "0026_activities_unique_constraint.sql" to db.go's
tableRebuildMigrations map so it gets the same autocommit-mode FK
disable/enable toggle (before/after the transaction) already used for
migrations 0023/0024/0025, and remove the now-redundant/misleading
mid-transaction PRAGMA lines from the migration file itself, matching
the established pattern.

Also restore the DEFAULT '' on event_type_key in migration 0026's
rebuilt activities table -- it was dropped from migration 0007's
original column definition during the rebuild, which broke every insert
that omits event_type_key and relies on that default
(TestClaimLegacyOwner_BindsExistingSingletonRowsToOneNewUser and others).

Extend TestRebuildMigrationsPreserveForeignKeyReferences to cover 0026:
seed a laps row and an activity_samples row (in addition to the existing
kind_assignments row) against a pre-existing activities row before the
rebuild migrations run, then verify after 0023-0026 complete that all
three child rows still exist and still reference the same activity, and
that FK enforcement rejects bogus activity_id/workout_kind_id afterward.
This is the regression guard that would have caught the original bug.

Note: TestClaimLegacyOwner_NoOpOnGenuinelyFreshInstall still fails on
this branch; verified it fails identically at the prior commit
(d8b7228), so it's a pre-existing, unrelated bug in ClaimLegacyOwner
(not migration 0026 or this FK-toggle bug) and out of scope for this fix.

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

backend/internal/store/migrations/0026_activities_unique_constraint.sql

@@ -4,13 +4,24 @@
 -- constraint alone was intentionally left in place by migration 0022 as a
 -- belts-and-suspenders safeguard (Garmin IDs are globally unique in practice),
 -- but it must be removed now to allow Task 6's cross-user test to pass.
-PRAGMA foreign_keys = OFF;
+--
+-- The FK enforcement toggle needed for this rebuild (DROP TABLE on a table
+-- with real inbound foreign keys) happens in Go code around this migration's
+-- execution (db.go's tableRebuildMigrations map), in autocommit mode before
+-- the transaction begins -- a mid-transaction PRAGMA foreign_keys statement
+-- is a documented no-op with modernc.org/sqlite, so it must not appear here.
+--
+-- event_type_key keeps the DEFAULT '' that migration 0007 originally gave
+-- it (ALTER TABLE ... ADD COLUMN event_type_key TEXT NOT NULL DEFAULT '');
+-- dropping the default here (as an earlier draft of this migration did)
+-- broke every INSERT that omits event_type_key and relies on that default,
+-- which several existing tests (e.g. TestClaimLegacyOwner_*) do.
 
 CREATE TABLE activities_new (
     id                         INTEGER PRIMARY KEY AUTOINCREMENT,
     user_id                    INTEGER REFERENCES users(id),
     garmin_activity_id         INTEGER NOT NULL,
-    event_type_key             TEXT NOT NULL,
+    event_type_key             TEXT NOT NULL DEFAULT '',
     workout_id                 INTEGER,
     start_time_utc             TEXT NOT NULL,
     duration_seconds           REAL NOT NULL,
@@ -42,5 +53,3 @@ ALTER TABLE activities_new RENAME TO activities;
 
 CREATE INDEX idx_activities_start_time ON activities(start_time_utc);
 CREATE INDEX idx_activities_user_garmin_id ON activities(user_id, garmin_activity_id);
-
-PRAGMA foreign_keys = ON;