feat: add scheduled scans (cron-like recurring scans)

- New `scheduled_scans` table with daily/weekly/monthly frequencies
- asyncio background scheduler loop checks for due schedules every 60s
- 6 REST endpoints: CRUD + toggle enabled + run-now
- `scheduled_scan_id` FK added to scans table; migrated automatically
- Frontend: Schedules page (list + create form), Schedules nav link,
  "Scheduled" badge on ScanDetails when scan was triggered by a schedule

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

flight-comparator/database/schema.sql

@@ -45,6 +45,9 @@ CREATE TABLE IF NOT EXISTS scans (
     seat_class TEXT DEFAULT 'economy',
     adults INTEGER DEFAULT 1 CHECK(adults > 0 AND adults <= 9),
 
+    -- FK to scheduled_scans (NULL for manual scans)
+    scheduled_scan_id INTEGER,
+
     -- Constraints across columns
     CHECK(end_date >= start_date),
     CHECK(routes_scanned <= total_routes OR total_routes = 0)
@@ -61,6 +64,10 @@ CREATE INDEX IF NOT EXISTS idx_scans_status
 CREATE INDEX IF NOT EXISTS idx_scans_created_at
     ON scans(created_at DESC);  -- For recent scans query
 
+CREATE INDEX IF NOT EXISTS idx_scans_scheduled_scan_id
+    ON scans(scheduled_scan_id)
+    WHERE scheduled_scan_id IS NOT NULL;
+
 -- ============================================================================
 -- Table: routes
 -- Purpose: Store discovered routes with flight statistics
@@ -244,7 +251,9 @@ ORDER BY created_at ASC;
 -- Initial Data: None (tables start empty)
 -- ============================================================================
 
+-- ============================================================================
 -- Schema version tracking (for future migrations)
+-- ============================================================================
 CREATE TABLE IF NOT EXISTS schema_version (
     version INTEGER PRIMARY KEY,
     applied_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
@@ -254,6 +263,64 @@ CREATE TABLE IF NOT EXISTS schema_version (
 INSERT OR IGNORE INTO schema_version (version, description)
 VALUES (1, 'Initial web app schema with scans and routes tables');
 
+-- ============================================================================
+-- Table: scheduled_scans
+-- Purpose: Define recurring scan schedules (daily / weekly / monthly)
+-- ============================================================================
+CREATE TABLE IF NOT EXISTS scheduled_scans (
+    id              INTEGER PRIMARY KEY AUTOINCREMENT,
+
+    -- Scan parameters (same as scans table)
+    origin          TEXT NOT NULL CHECK(length(origin) = 3),
+    country         TEXT NOT NULL CHECK(length(country) >= 2),
+    window_months   INTEGER NOT NULL DEFAULT 1
+                        CHECK(window_months >= 1 AND window_months <= 12),
+    seat_class      TEXT NOT NULL DEFAULT 'economy',
+    adults          INTEGER NOT NULL DEFAULT 1
+                        CHECK(adults > 0 AND adults <= 9),
+
+    -- Schedule definition
+    frequency       TEXT NOT NULL
+                        CHECK(frequency IN ('daily', 'weekly', 'monthly')),
+    hour            INTEGER NOT NULL DEFAULT 6
+                        CHECK(hour >= 0 AND hour <= 23),
+    minute          INTEGER NOT NULL DEFAULT 0
+                        CHECK(minute >= 0 AND minute <= 59),
+    day_of_week     INTEGER CHECK(day_of_week >= 0 AND day_of_week <= 6),
+    day_of_month    INTEGER CHECK(day_of_month >= 1 AND day_of_month <= 28),
+
+    -- State
+    enabled         INTEGER NOT NULL DEFAULT 1,
+    label           TEXT,
+    last_run_at     TIMESTAMP,
+    next_run_at     TIMESTAMP NOT NULL,
+
+    created_at      TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    updated_at      TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    -- Frequency-specific field requirements
+    CHECK(
+        (frequency = 'weekly'  AND day_of_week  IS NOT NULL) OR
+        (frequency = 'monthly' AND day_of_month IS NOT NULL) OR
+        (frequency = 'daily')
+    )
+);
+
+-- Fast lookup of due schedules (partial index on enabled rows only)
+CREATE INDEX IF NOT EXISTS idx_scheduled_scans_next_run
+    ON scheduled_scans(next_run_at)
+    WHERE enabled = 1;
+
+-- Auto-update updated_at on every PATCH
+CREATE TRIGGER IF NOT EXISTS update_scheduled_scans_timestamp
+AFTER UPDATE ON scheduled_scans
+FOR EACH ROW BEGIN
+    UPDATE scheduled_scans SET updated_at = CURRENT_TIMESTAMP WHERE id = NEW.id;
+END;
+
+INSERT OR IGNORE INTO schema_version (version, description)
+VALUES (2, 'Add scheduled_scans table');
+
 -- ============================================================================
 -- Verification Queries (for testing)
 -- ============================================================================