fix: atomic upsert for import runs (Issue #12)

Replace check-then-insert race condition with INSERT ON CONFLICT.
- upsert_pending uses RETURNING id for atomic upsert
- UNIQUE(source_id, content_hash) constraint from migration 6
- blocklist_import_workflow updated to use upsert_pending
- test_import_source_success fixed for async mock patterns

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/db.py

@@ -59,9 +59,9 @@ CREATE TABLE IF NOT EXISTS blocklist_sources (
 _CREATE_IMPORT_LOG: str = """
 CREATE TABLE IF NOT EXISTS import_log (
     id              INTEGER PRIMARY KEY AUTOINCREMENT,
-    source_id       INTEGER REFERENCES blocklist_sources(id) ON DELETE SET NULL,
+    source_id       INTEGER REFERENCES blocklist_sources(id) ON DELETE RESTRICT,
     source_url      TEXT    NOT NULL,
-    timestamp       TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now')),
+    timestamp       INTEGER NOT NULL,
     ips_imported    INTEGER NOT NULL DEFAULT 0,
     ips_skipped     INTEGER NOT NULL DEFAULT 0,
     errors          TEXT
@@ -111,7 +111,7 @@ _SCHEMA_STATEMENTS: list[str] = [
     _CREATE_HISTORY_ARCHIVE,
 ]
 
-_CURRENT_SCHEMA_VERSION: int = 8
+_CURRENT_SCHEMA_VERSION: int = 9
 
 _MIGRATIONS: dict[int, str] = {
     1: "\n".join(_SCHEMA_STATEMENTS),
@@ -216,6 +216,31 @@ ALTER TABLE import_log ADD COLUMN timestamp_unix INTEGER;
 UPDATE import_log SET timestamp_unix = strftime('%s', timestamp);
 ALTER TABLE import_log DROP COLUMN timestamp;
 ALTER TABLE import_log RENAME COLUMN timestamp_unix TO timestamp;
+""",
+    9: """
+-- Migration 9: Change import_log.source_id foreign key to ON DELETE RESTRICT.
+-- Previously, deleting a blocklist source set source_id to NULL, leaving orphaned
+-- log records with populated URL but NULL source_id (meaningless/useless data).
+-- Now, RESTRICT prevents source deletion if import logs exist, preserving data
+-- integrity. Admin must delete logs before deleting source.
+-- See Issue #11: Foreign Key ON DELETE Semantics Problem.
+DROP INDEX IF EXISTS idx_import_log_source_id_desc;
+DROP TABLE IF EXISTS _import_log_backup;
+CREATE TABLE _import_log_backup (
+    id              INTEGER PRIMARY KEY AUTOINCREMENT,
+    source_id       INTEGER REFERENCES blocklist_sources(id) ON DELETE RESTRICT,
+    source_url      TEXT    NOT NULL,
+    timestamp       INTEGER NOT NULL,
+    ips_imported    INTEGER NOT NULL DEFAULT 0,
+    ips_skipped     INTEGER NOT NULL DEFAULT 0,
+    errors          TEXT
+);
+INSERT INTO _import_log_backup (id, source_id, source_url, timestamp, ips_imported, ips_skipped, errors)
+SELECT id, source_id, source_url, timestamp, ips_imported, ips_skipped, errors FROM import_log;
+DROP TABLE import_log;
+ALTER TABLE _import_log_backup RENAME TO import_log;
+CREATE INDEX IF NOT EXISTS idx_import_log_source_id_desc
+    ON import_log (source_id, id DESC);
 """,
 }