feat(#384): Add Synapse + Element Web to docker-compose for dev

- Create docker-compose.matrix.yml as optional dev overlay
- Add Synapse homeserver config with shared PostgreSQL
- Add Element Web client config (port 8501)
- Add bot account setup script (docker/matrix/scripts/setup-bot.sh)
- Add Makefile targets: matrix-up, matrix-down, matrix-logs, matrix-setup-bot
- Document Matrix env vars in .env.example
- Synapse accessible at localhost:8008, Element at localhost:8501
- Usage: docker compose -f docker/docker-compose.yml -f docker/docker-compose.matrix.yml up

Refs #384

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

docker/matrix/synapse/homeserver.yaml

@@ -0,0 +1,131 @@
+# ==============================================
+# Synapse Homeserver Configuration — Development Only
+# ==============================================
+#
+# This config is for LOCAL DEVELOPMENT with the Mosaic Stack docker-compose overlay.
+# Do NOT use this in production. See docker-compose.sample.matrix.yml for production.
+#
+# Server name is set to 'localhost' — this is permanent and cannot be changed
+# after the database has been initialized.
+#
+# ==============================================
+
+server_name: "localhost"
+pid_file: /data/homeserver.pid
+public_baseurl: "http://localhost:8008/"
+
+# ======================
+# Network Listeners
+# ======================
+listeners:
+  # Client API (used by Element Web, Mosaic bridge, etc.)
+  - port: 8008
+    tls: false
+    type: http
+    x_forwarded: true
+    bind_addresses: ["0.0.0.0"]
+    resources:
+      - names: [client, federation]
+        compress: false
+
+# ======================
+# Database (Shared PostgreSQL)
+# ======================
+database:
+  name: psycopg2
+  txn_limit: 10000
+  args:
+    user: "synapse"
+    password: "synapse_dev_password"
+    database: "synapse"
+    host: "postgres"
+    port: 5432
+    cp_min: 5
+    cp_max: 10
+
+# ======================
+# Media Storage
+# ======================
+media_store_path: /data/media_store
+max_upload_size: 50M
+url_preview_enabled: true
+url_preview_ip_range_blacklist:
+  - "127.0.0.0/8"
+  - "10.0.0.0/8"
+  - "172.16.0.0/12"
+  - "192.168.0.0/16"
+  - "100.64.0.0/10"
+  - "192.0.0.0/24"
+  - "169.254.0.0/16"
+  - "198.18.0.0/15"
+  - "::1/128"
+  - "fe80::/10"
+  - "fc00::/7"
+  - "2001:db8::/32"
+  - "ff00::/8"
+  - "fec0::/10"
+
+# ======================
+# Registration (Dev Only)
+# ======================
+enable_registration: true
+enable_registration_without_verification: true
+
+# ======================
+# Signing Keys
+# ======================
+# Auto-generated on first startup and persisted in the signing_key volume
+signing_key_path: "/data/keys/localhost.signing.key"
+
+# Suppress warning about trusted key servers in dev
+suppress_key_server_warning: true
+trusted_key_servers: []
+
+# ======================
+# Room Configuration
+# ======================
+enable_room_list_search: true
+allow_public_rooms_over_federation: false
+
+# ======================
+# Rate Limiting (Relaxed for Dev)
+# ======================
+rc_message:
+  per_second: 100
+  burst_count: 200
+
+rc_registration:
+  per_second: 10
+  burst_count: 50
+
+rc_login:
+  address:
+    per_second: 10
+    burst_count: 50
+  account:
+    per_second: 10
+    burst_count: 50
+
+# ======================
+# Logging
+# ======================
+log_config: "/data/localhost.log.config"
+
+# Inline log config — write to stdout for docker logs
+# Synapse falls back to a basic console logger if the log_config file is missing,
+# so we leave log_config pointing to a non-existent file intentionally.
+# Override: mount a custom log config file at /data/localhost.log.config
+
+# ======================
+# Miscellaneous
+# ======================
+report_stats: false
+macaroon_secret_key: "dev-macaroon-secret-change-in-production"
+form_secret: "dev-form-secret-change-in-production"
+
+# Enable presence for dev
+use_presence: true
+
+# Retention policy (optional, keep messages for 180 days in dev)
+retention:
+  enabled: false