Add MCP server with OAuth/API-key auth (#6570)

Adds an optional MCP server (proprietary module) that exposes Stirling's
PDF operations and AI capabilities to MCP clients. Off by default, zero
footprint when disabled.

### What
- New `/mcp` endpoint: streamable-HTTP + JSON-RPC 2.0; 8 tools
(describe_operation, pages/convert/misc/security category tools, AI,
upload, download).
- Runs real operations over an internal loopback; results returned
inline as base64 (small) or by fileId (large).

### Auth (two modes)
- OAuth2 resource server: RFC 9728 protected-resource metadata, RFC 8707
audience binding, JWKS, `mcp.tools.read/write` scopes; binds each token
to a provisioned Stirling account.
- API-key mode: reuses Stirling per-user `X-API-KEY` (no IdP needed).

### Security
- Per-user file ownership in FileStorage: async/queued writes scoped to
the submitting user; legacy/owner-less files stay readable.
- Admin allow/block list controls which operations are exposed.
- Python engine gated behind a shared secret (`X-Engine-Auth`).
- MCP filter chain is isolated and cannot weaken the main app's
security.
- Hardened: no upstream error-body leakage, log injection sanitized,
fileId path/sidecar enumeration blocked.

### Config / footprint
- Disabled by default (`mcp.enabled=false`); all beans
`@ConditionalOnProperty`.
---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

app/core/src/main/resources/settings.yml.template

@@ -364,6 +364,24 @@ aiEngine:
   url: http://localhost:5001 # URL of the Python AI engine
   timeoutSeconds: 120 # Timeout in seconds for AI engine requests
 
+# Model Context Protocol (MCP) server. Exposes Stirling's PDF tools (grouped by namespace)
+# plus the AI agents to MCP clients (Inspector, Claude Desktop, custom). OAuth-protected.
+# Disabled by default - enable explicitly per deployment after configuring mcp.auth.
+mcp:
+  enabled: false # Master switch. 'false' (default) means no /mcp endpoint, no metadata, no beans wired.
+  scopesEnabled: true # Enforce mcp.tools.read / mcp.tools.write scopes derived from operation category
+  allowedOperations: [] # Tool allow-list (operation ids, e.g. ['compress-pdf']). Empty = all. When set, ONLY these are exposed over MCP.
+  blockedOperations: [] # Tool deny-list (operation ids). Always removed from MCP even if otherwise allowed.
+  auth:
+    mode: oauth # 'oauth' (full OAuth2 resource server) or 'apikey' (Stirling per-user API key via X-API-KEY header; no external IdP needed - the low-friction self-host option)
+    issuerUri: "" # OAuth2 issuer URI (e.g. http://localhost:9000). Required when mode=oauth.
+    jwksUri: "" # JWKS URI. Blank -> derived from issuer's /.well-known/openid-configuration.
+    resourceId: "" # RFC 8707 resource identifier of THIS MCP server (e.g. http://localhost:8080/mcp).
+                   # Required: tokens must list this id in `aud` or the request is rejected.
+    usernameClaim: sub # JWT claim matched against a Stirling username (e.g. 'sub', 'email', 'preferred_username')
+    requireExistingAccount: true # Reject tokens whose subject has no enabled Stirling account (recommended)
+  engineCapabilityRefreshMinutes: 5 # How often to refresh the AI capabilities manifest from the engine
+
 # Cluster configuration. NOT YET ENABLED - scaffolding for later work. Leave at defaults.
 cluster:
   enabled: false # Master switch. 'false' (default) wires the in-process backplane and skips all cluster checks. Single-instance installs do not need to change anything here.