feat: implement reverse scan (country → specific airports)

- DB schema: relaxed origin CHECK to >=2 chars, added scan_mode column to
  scans and scheduled_scans, added origin_airport to routes and flights,
  updated unique index to (scan_id, COALESCE(origin_airport,''), destination)
- Migrations: init_db.py recreates tables and adds columns via guarded ALTERs
- API: scan_mode field on ScanRequest/Scan; Route/Flight expose origin_airport;
  GET /scans/{id}/flights accepts origin_airport filter; CreateScheduleRequest
  and Schedule carry scan_mode; scheduler and run-now pass scan_mode through
- scan_processor: _write_route_incremental accepts origin_airport; process_scan
  branches on scan_mode=reverse (country → airports × destinations × dates)
- Frontend: new CountrySelect component (populated from GET /api/v1/countries);
  Scans page adds Direction toggle + CountrySelect for both modes; ScanDetails
  shows Origin column for reverse scans and uses composite route keys; Re-run
  preserves scan_mode

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

flight-comparator/database/schema.sql

@@ -20,8 +20,11 @@ CREATE TABLE IF NOT EXISTS scans (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
 
     -- Search parameters (validated by CHECK constraints)
-    origin TEXT NOT NULL CHECK(length(origin) = 3),
+    -- origin stores IATA code (forward scans) or ISO country code (reverse scans)
+    origin TEXT NOT NULL CHECK(length(origin) >= 2),
     country TEXT NOT NULL CHECK(length(country) >= 2),
+    scan_mode TEXT NOT NULL DEFAULT 'forward'
+        CHECK(scan_mode IN ('forward', 'reverse')),
     start_date TEXT NOT NULL,  -- ISO 8601: YYYY-MM-DD
     end_date TEXT NOT NULL,
 
@@ -81,7 +84,10 @@ CREATE TABLE IF NOT EXISTS routes (
     -- Foreign key to scans (cascade delete)
     scan_id INTEGER NOT NULL,
 
-    -- Destination airport
+    -- Route airports
+    -- For forward scans: origin_airport is NULL (implicit from scan.origin)
+    -- For reverse scans: origin_airport is the variable origin IATA
+    origin_airport TEXT,
     destination TEXT NOT NULL CHECK(length(destination) = 3),
     destination_name TEXT NOT NULL,
     destination_city TEXT,
@@ -120,9 +126,9 @@ CREATE INDEX IF NOT EXISTS idx_routes_min_price
     ON routes(min_price)
     WHERE min_price IS NOT NULL;  -- Partial index for routes with prices
 
--- One route row per (scan, destination) — enables incremental upsert writes
-CREATE UNIQUE INDEX IF NOT EXISTS uq_routes_scan_dest
-    ON routes(scan_id, destination);
+-- One route row per (scan, origin_airport, destination) — supports both forward and reverse scans
+CREATE UNIQUE INDEX IF NOT EXISTS uq_routes_scan_origin_dest
+    ON routes(scan_id, COALESCE(origin_airport, ''), destination);
 
 -- ============================================================================
 -- Triggers: Auto-update timestamps and aggregates
@@ -191,6 +197,8 @@ CREATE TABLE IF NOT EXISTS flights (
     scan_id INTEGER NOT NULL,
 
     -- Route
+    -- origin_airport: NULL for forward scans, specific IATA for reverse scans
+    origin_airport TEXT,
     destination TEXT NOT NULL CHECK(length(destination) = 3),
     date TEXT NOT NULL,  -- ISO 8601: YYYY-MM-DD
 
@@ -273,8 +281,10 @@ 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),
+    origin          TEXT NOT NULL CHECK(length(origin) >= 2),
     country         TEXT NOT NULL CHECK(length(country) >= 2),
+    scan_mode       TEXT NOT NULL DEFAULT 'forward'
+                        CHECK(scan_mode IN ('forward', 'reverse')),
     window_months   INTEGER NOT NULL DEFAULT 1
                         CHECK(window_months >= 1 AND window_months <= 12),
     seat_class      TEXT NOT NULL DEFAULT 'economy',