fix(security): deny generated-image access when ownership verification fails (#6268)

* fix(security): deny generated-image access when ownership verification fails

The except Exception block in serve_generated_image catches all
exceptions during ownership verification and falls through to serve
the file anyway. If the database is down or corrupted, the ownership
check is bypassed entirely — any authenticated user can download any
generated image by guessing the content-hash filename.

Raise HTTP 503 (Service Unavailable) instead of falling through,
so the endpoint fails closed when verification is impossible. This
matches the defense-in-depth principle: deny access on verification
errors rather than granting it.

* fix(security): enforce fail-closed ownership and secure caching for generated images

- Fail closed on missing gallery records, null/empty owners, and DB errors
- Support both cookie session and bearer API token owners via effective_user
- Add in-memory registry for freshly generated images to preserve access
- Eliminate file existence oracle by verifying ownership before disk probing
- Update Cache-Control to private, no-cache, must-revalidate
- Pass owner attribution through tool execution and image MCP server
- Add comprehensive hermetic regression tests for ownership gates and caching

* fix(security): sanitize tool owner arguments and drop unneeded MCP env fallback

- Remove redundant and unsafe ODYSSEUS_MCP_IMAGE_OWNER fallback in image_gen_server.py to fix 4 tests in test_env_reference.py.
- Sanitize caller-supplied tool arguments across all MCP and email tool dispatch paths in src/tool_execution.py: strip untrusted _odysseus_* keys from arguments.
- Only inject _odysseus_owner when verified trusted server context is present, preventing untrusted/anonymous users from spoofing ownership claims.
- Sanitize empty/whitespace owner strings in image_gen_server.py to None.
- Add comprehensive regression tests in test_generated_image_ownership.py for argument sanitization, owner injection, fallback propagation, and server sanitization.

---------

Co-authored-by: isharak7m <192635824+isharak7m@users.noreply.github.com>
Co-authored-by: PR #6166 Reviewer <reviewer@odysseus-review.local>

tests/test_generated_image_confinement.py

@@ -60,7 +60,7 @@ def test_generated_image_headers_include_nosniff():
     assert generated_images.GENERATED_IMAGE_HEADERS["X-Content-Type-Options"] == "nosniff"
     assert (
         generated_images.GENERATED_IMAGE_HEADERS["Cache-Control"]
-        == "public, max-age=31536000, immutable"
+        == "private, no-cache, must-revalidate"
     )