Make Go module uploads immutable

[philipgreat]

Fixes #4.

Go module artifact paths are now create-only. Known duplicates return HTTP 409 before blob creation. A PostgreSQL unique index on non-empty asset paths closes concurrent PUT races, and startup reports pre-existing duplicate paths before attempting the index.

Verification on a dedicated PostgreSQL test database:

• cargo check -p teaql-registry --tests --locked
• cargo fmt --all --check
• cargo test -p teaql-registry --test multiformat_test --locked -- --test-threads=1: 9 passed, 0 failed
• Duplicate mod, zip and info uploads return 409; original downloaded bytes remain unchanged.
• Duplicate requests produce no replacement blob metadata.
• Concurrent PUTs produce one 201 and one 409.
• Dedicated test database removed after verification.

Deployment gate: audit existing asset path duplicates before the first restart. The currently running Registry container was not rebuilt or restarted, and HANA remained stopped.

[philipgreat]

Base corrected to test/masking-registry-gate, the actual parent of this fix. The PR now contains only the six files for issue #4 and is mergeable. The current CI workflow filters pull_request to main/master, so this test-branch PR has no GitHub checks; it must not be described as CI green.

Local isolated PostgreSQL gate at exact HEAD 7337f8b:

• cargo fmt --all --check: PASS
• cargo check -p teaql-registry --tests --locked: PASS
• cargo clippy -p teaql-registry -p registry-tui --all-targets --no-deps --locked -- -D warnings: PASS
• cargo test --locked -- --test-threads=1: exit 0; all test binaries and doc-tests passed (84 executed tests, zero failures)
• Unique asset path index present after tests; duplicate non-empty asset path count zero.

The dedicated test database was removed after validation. No running Registry container was rebuilt or restarted; HANA remained stopped. Integration into main and deployment remain separate gates.