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

certs/.gitignore

@@ -0,0 +1,6 @@
+# Ignore all cert/key files EXCEPT the bundled snakeoil development defaults.
+# Never commit real TLS certificates or private keys to version control.
+*
+!.gitignore
+!snakeoil.crt
+!snakeoil.key

certs/snakeoil.crt

@@ -0,0 +1,22 @@
+-----BEGIN CERTIFICATE-----
+MIIDoTCCAomgAwIBAgIUPgupZzf+zgMp4ylvf1NBRIWNpeUwDQYJKoZIhvcNAQEL
+BQAwUjELMAkGA1UEBhMCR0IxDjAMBgNVBAgMBUxvY2FsMQ4wDAYDVQQHDAVMb2Nh
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+o59gDmbCXhypom7OsyxBLZgyVWU1
+-----END CERTIFICATE-----

certs/snakeoil.key

@@ -0,0 +1,28 @@
+-----BEGIN PRIVATE KEY-----
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+-----END PRIVATE KEY-----