fix(security): comprehensive security audit and hardening

Auth: login rate limit 30/min (was 500), global rate limit 1000/min (was
unlimited), password/username max lengths on all Zod schemas, session
invalidation on role change, API key legacy scan bounded to 100 keys.

SVG: hardened regex sanitizer with CDATA stripping, XML entity decoding,
set/animate/iframe/embed blocking, comprehensive data: URI blocking,
use element external href blocking. 11 attack payload fixtures added.

SSRF: fixed DNS rebinding TOCTOU by pinning resolved IPs via custom
HTTP/HTTPS agents. Added 6to4 and NAT64 to blocked IPv6 ranges.

Docker: capability dropping (cap_drop ALL + minimal cap_add), resource
limits (4g/8g mem, 512/1024 pids), healthcheck timeout, password
removed from startup banner, default password warning comments.

Network: CSP and HSTS applied in all environments (not just production),
stack traces removed from all error responses, internal paths stripped
from error details, per-route rate limits on uploads (60/min) and URL
fetches (200/hour).

Files: exclusive temp file creation (O_EXCL), disk space circuit
breaker, per-user storage quotas, settings payload 64KB size guard.

Python sidecar: script name allowlist in dispatcher, minimal environment
for subprocess spawns.

Dependencies: fixed 6 production CVEs (drizzle-orm, fastify, fast-uri,
@fastify/static, next, archiver/lodash). Pinned all GitHub Actions to
SHA hashes.

114 security tests added. Full OWASP Top 10 penetration test matrix
verified against production Docker container (30/30 pass after
hardening).

docker/docker-compose-gpu.yml

@@ -13,6 +13,8 @@ services:
     image: snapotter:latest
     container_name: SnapOtter
     ports:
+      # For internet-facing deployments, bind to localhost only:
+      # - "127.0.0.1:1349:1349"
       - "1349:1349"
     volumes:
       - SnapOtter-data:/data           # Database, AI models, user files
@@ -20,6 +22,8 @@ services:
     environment:
       - AUTH_ENABLED=true
       - DEFAULT_USERNAME=admin
+      # Set a strong password. Default is 'admin' -- CHANGE THIS for any non-local deployment.
+      # - DEFAULT_PASSWORD=your-strong-password-here
       - DEFAULT_PASSWORD=admin
       - SKIP_MUST_CHANGE_PASSWORD=${SKIP_MUST_CHANGE_PASSWORD:-false}
       - MAX_UPLOAD_SIZE_MB=${MAX_UPLOAD_SIZE_MB:-0}
@@ -34,6 +38,25 @@ services:
       - SESSION_DURATION_HOURS=${SESSION_DURATION_HOURS:-168}
       - TRUST_PROXY=${TRUST_PROXY:-true}
     restart: unless-stopped
+    # --- Security hardening ---
+    mem_limit: 8g
+    memswap_limit: 8g
+    cpus: 8
+    pids_limit: 1024
+    cap_drop:
+      - ALL
+    cap_add:
+      - CHOWN
+      - SETUID
+      - SETGID
+      - DAC_OVERRIDE
+      - FOWNER
+    # NOTE: security_opt: [no-new-privileges:true] is intentionally omitted.
+    # gosu requires setuid to drop from root to the snapotter user.
+    # Mitigation: cap_drop: ALL limits available capabilities after privilege drop.
+    # NOTE: read_only: true is not set because PUID/PGID remapping requires
+    # writing to /etc/passwd and /etc/group. Consider using Docker --user flag
+    # instead of PUID/PGID for read-only rootfs support.
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:1349/api/v1/health"]
       interval: 30s