Feature: Walk-Einladungen und RSVP-System

Gassi-Treffen bekommen ein vollständiges Einladungs- und RSVP-System:
- Neue Tabelle walk_invitations (walk_id, user_id, status, Zeitstempel)
- Backend: /invite-candidates, /invite, /rsvp, /participants Endpoints
- Push-Notification beim Einladen
- Frontend: RSVP-Buttons (Zusagen/Vielleicht/Absagen), Teilnehmerliste
  mit Avatar-Initialen und farbkodierten RSVP-Badges, Einladen-Modal
- SW by-v205, APP_VER 173

backend/database.py

@@ -464,6 +464,8 @@ def _migrate(conn_factory):
         ("dogs", "foto_zoom",     "REAL NOT NULL DEFAULT 1.0"),
         ("dogs", "foto_offset_x", "REAL NOT NULL DEFAULT 0.0"),
         ("dogs", "foto_offset_y", "REAL NOT NULL DEFAULT 0.0"),
+        # Tagebuch: Ortsname (POI/Adresse)
+        ("diary", "location_name", "TEXT"),
     ]
     with conn_factory() as conn:
         for table, column, col_type in migrations:
@@ -736,3 +738,19 @@ def _migrate(conn_factory):
             CREATE INDEX IF NOT EXISTS idx_service_offers_type  ON service_offers(type, aktiv);
             CREATE INDEX IF NOT EXISTS idx_service_offers_user  ON service_offers(user_id, type);
         """)
+
+        # Walk-Einladungen (RSVP)
+        conn.executescript("""
+            CREATE TABLE IF NOT EXISTS walk_invitations (
+                id           INTEGER PRIMARY KEY AUTOINCREMENT,
+                walk_id      INTEGER NOT NULL REFERENCES walks(id) ON DELETE CASCADE,
+                user_id      INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+                status       TEXT    NOT NULL DEFAULT 'invited',
+                invited_at   TEXT    NOT NULL DEFAULT (datetime('now')),
+                responded_at TEXT,
+                UNIQUE(walk_id, user_id)
+            );
+            CREATE INDEX IF NOT EXISTS idx_walk_inv_walk ON walk_invitations(walk_id);
+            CREATE INDEX IF NOT EXISTS idx_walk_inv_user ON walk_invitations(user_id, status);
+        """)
+        logger.info("Migration: walk_invitations Tabelle bereit.")