chore: add platform identity and platform_messages for multi-platform support (#23)

TDD cycle for issue #23:
- RED: 9 migration smoke tests (file presence + schema expectations)
- GREEN: V016 migration adding platform identity columns
- GREEN: CreateSessionHandler, JoinSessionHandler, Web SessionService updated
  with dual-write to legacy and new identity columns + COALESCE fallbacks
- GREEN: get_group_attendance_stats recreated for external_username
- Bump version to 2.0.0

Changes:
- V016__add_platform_identity.sql:
  - players: platform, external_user_id, external_username
  - game_groups: platform, external_group_id, external_channel_id
  - platform_messages table with cross-platform message tracking
  - Backfill all existing Telegram data into new columns
  - Recreate get_group_attendance_stats with COALESCE fallback
- V012__add_attendance_stats.sql: use COALESCE(external_username, telegram_username)
- CreateSessionHandler: dual-write + COALESCE fallbacks in SELECTs
- JoinSessionHandler: dual-write to new identity columns
- Web SessionService: dual-write to new identity columns
- PlatformIdentityMigrationTests (9 smoke tests covering all handlers)
- Version synced: Directory.Build.props, compose.yaml, deploy.yml, NavMenu.razor → 2.0.0

Legacy telegram_* columns preserved for backward compatibility.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Bot/Migrations/V016__add_platform_identity.sql

@@ -0,0 +1,119 @@
+-- =============================================================
+-- V016: Add platform identity columns and platform_messages table
+-- =============================================================
+-- Scope: Prepare schema for multi-platform support (Discord, etc).
+-- Legacy telegram_* columns are retained for backward compatibility.
+-- =============================================================
+
+-- -- Players: platform-agnostic identity
+ALTER TABLE players
+    ADD COLUMN platform          VARCHAR(50),
+    ADD COLUMN external_user_id  VARCHAR(255),
+    ADD COLUMN external_username VARCHAR(255);
+
+CREATE UNIQUE INDEX ix_players_platform_external_user_id
+    ON players (platform, external_user_id)
+    WHERE platform IS NOT NULL AND external_user_id IS NOT NULL;
+
+-- -- Game groups: platform-agnostic identity
+ALTER TABLE game_groups
+    ADD COLUMN platform           VARCHAR(50),
+    ADD COLUMN external_group_id  VARCHAR(255),
+    ADD COLUMN external_channel_id VARCHAR(255);
+
+CREATE UNIQUE INDEX ix_game_groups_platform_external_group_id
+    ON game_groups (platform, external_group_id)
+    WHERE platform IS NOT NULL AND external_group_id IS NOT NULL;
+
+-- -- Backfill existing Telegram data
+UPDATE players
+   SET platform          = 'Telegram',
+       external_user_id  = telegram_id::TEXT,
+       external_username = telegram_username
+ WHERE platform IS NULL;
+
+UPDATE game_groups
+   SET platform          = 'Telegram',
+       external_group_id = telegram_chat_id::TEXT
+ WHERE platform IS NULL;
+
+-- -- Platform messages: store per-platform message references
+CREATE TABLE platform_messages (
+    id                   UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    platform             VARCHAR(50) NOT NULL,
+    group_id             UUID REFERENCES game_groups(id) ON DELETE CASCADE,
+    batch_id             UUID,
+    session_id           UUID REFERENCES sessions(id) ON DELETE CASCADE,
+    external_channel_id  VARCHAR(255),
+    external_thread_id   VARCHAR(255),
+    external_message_id  VARCHAR(255) NOT NULL,
+    purpose              VARCHAR(50) NOT NULL,
+    created_at           TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX ix_platform_messages_group_id     ON platform_messages(group_id);
+CREATE INDEX ix_platform_messages_batch_id     ON platform_messages(batch_id);
+CREATE INDEX ix_platform_messages_session_id   ON platform_messages(session_id);
+CREATE INDEX ix_platform_messages_platform_message
+    ON platform_messages (platform, external_message_id);
+
+-- -- Recreate attendance stats function for new columns (prod back-compat)
+CREATE OR REPLACE FUNCTION get_group_attendance_stats(p_group_id UUID)
+RETURNS TABLE (
+    player_id           UUID,
+    display_name        VARCHAR,
+    telegram_username   VARCHAR,
+    total_sessions      BIGINT,
+    confirmed_count     BIGINT,
+    declined_count      BIGINT,
+    no_response_count   BIGINT,
+    waitlisted_count    BIGINT,
+    cancellation_affected_count BIGINT,
+    attendance_rate     NUMERIC
+) AS $$
+BEGIN
+    RETURN QUERY
+    WITH player_sessions AS (
+        SELECT
+            sp.player_id,
+            s.id                AS session_id,
+            sp.rsvp_status,
+            sp.registration_status,
+            s.status            AS session_status,
+            s.scheduled_at
+        FROM session_participants sp
+        JOIN sessions s ON s.id = sp.session_id
+        WHERE s.group_id = p_group_id
+    ),
+    player_totals AS (
+        SELECT
+            ps.player_id,
+            COUNT(*) FILTER (WHERE ps.session_status <> 'Cancelled')                      AS total_sessions,
+            COUNT(*) FILTER (WHERE ps.rsvp_status = 'Confirmed' AND ps.session_status <> 'Cancelled') AS confirmed_count,
+            COUNT(*) FILTER (WHERE ps.rsvp_status = 'Declined' AND ps.session_status <> 'Cancelled')  AS declined_count,
+            COUNT(*) FILTER (WHERE ps.rsvp_status = 'Pending' AND ps.scheduled_at < NOW() AND ps.session_status <> 'Cancelled') AS no_response_count,
+            COUNT(*) FILTER (WHERE ps.registration_status = 'Waitlisted' AND ps.session_status <> 'Cancelled') AS waitlisted_count,
+            COUNT(*) FILTER (WHERE ps.session_status = 'Cancelled')                        AS cancellation_affected_count
+        FROM player_sessions ps
+        GROUP BY ps.player_id
+    )
+    SELECT
+        pt.player_id,
+        p.display_name,
+        COALESCE(p.external_username, p.telegram_username) AS telegram_username,
+        pt.total_sessions,
+        pt.confirmed_count,
+        pt.declined_count,
+        pt.no_response_count,
+        pt.waitlisted_count,
+        pt.cancellation_affected_count,
+        ROUND(
+            100.0 * pt.confirmed_count
+            / NULLIF(pt.total_sessions, 0),
+            1
+        ) AS attendance_rate
+    FROM player_totals pt
+    JOIN players p ON p.id = pt.player_id
+    ORDER BY pt.confirmed_count DESC, pt.total_sessions DESC;
+END;
+$$ LANGUAGE plpgsql STABLE;