feat: native HTTPS support with bundled snakeoil default cert

server/index.js:
- Import http and https modules
- Resolve TLS_ENABLED early (before Helmet) so upgradeInsecureRequests
  CSP directive fires when TLS is active directly (not only via proxy)
- loadTlsCredentials() reads TLS_CERT/TLS_KEY (defaulting to bundled
  snakeoil) and returns null on failure (graceful HTTP fallback)
- Start https.createServer or http.createServer depending on credentials
- Startup banner now shows protocol, TLS cert path, and snakeoil warning

certs/:
- Add bundled snakeoil self-signed certificate (RSA 2048, 10yr, SAN for
  localhost + 127.0.0.1) for out-of-the-box HTTPS without configuration
- .gitignore allows only snakeoil.{crt,key} — real certs must not be
  committed

Dockerfile:
- COPY certs/ into image so snakeoil default is always available
- HEALTHCHECK updated to https:// with --no-check-certificate

docker-compose.yaml:
- Port now exposes HTTPS directly by default
- TLS_CERT/TLS_KEY/TLS_ENABLED/TRUST_PROXY documented with Option A/B
- cert volume mount examples added (commented out)
- healthcheck updated to https with --no-check-certificate

.env.sample:
- New TLS/HTTPS section with TLS_ENABLED, TLS_CERT, TLS_KEY
- openssl self-signed cert generation example included

docs/ARCHITECTURE.md:
- Configuration table: TLS_ENABLED, TLS_CERT, TLS_KEY env vars added
- Docker image section: TLS default behaviour documented
- Docker Compose example: Option A (direct TLS) / Option B (proxy) layout
- Security checklist: HTTPS now first item, updated for TLS modes

.env.sample

@@ -19,6 +19,24 @@ LOG_LEVEL=info
 # Generate with: openssl rand -hex 32
 COOKIE_SECRET=your-cookie-secret-here
 
+# =============================================================================
+# TLS / HTTPS
+# =============================================================================
+
+# TLS is enabled by default using the bundled snakeoil self-signed certificate
+# (valid for localhost/127.0.0.1, 10-year expiry).
+# Set TLS_CERT and TLS_KEY to use your own certificate (recommended).
+# Set TLS_ENABLED=false to run in plain HTTP mode (e.g. behind a TLS proxy).
+#
+# To generate a self-signed cert for your own hostname:
+#   openssl req -x509 -newkey rsa:2048 -keyout server.key -out server.crt \
+#     -days 365 -nodes -subj "/CN=yourhostname" \
+#     -addext "subjectAltName=DNS:yourhostname,IP:192.168.x.x"
+#
+# TLS_ENABLED=true
+# TLS_CERT=/path/to/server.crt
+# TLS_KEY=/path/to/server.key
+
 # =============================================================================
 # REVERSE PROXY & DEPLOYMENT
 # =============================================================================