[spelech]

Fixes #78.

Changes

• Propagate vector initialization/deletion/upsert failures instead of returning success and positive indexed chunk counts.
• Reuse existing disk backup/rollback. Invalidate previous relational index metadata before destructive vector replacement so a restored file can be retried rather than falsely considered indexed.
• Attempt cleanup of partially persisted batches and document the retry policy; no distributed-transaction guarantee.
• Remove old vectors on a successful empty replacement.

Verification

• New real SQLite/Qdrant/filesystem/HTTP regressions: 7 passed after reproducing 5 expected failures.
• Generated requirement inventory: 2 passed; release/link-integrity gate passed.
• Combined backend regression run: 70 passed, 1 skipped.
• With the companion PostgreSQL fixes: 2 authenticated full-profile checks passed. A real PostgreSQL trigger rejected vector inserts; uploads returned HTTP 500, original PDF bytes were restored, stale index/cache records were absent and retry succeeded after removing the fault.

Broader storage checks had 21 passes and one unchanged cached-runtime failure: tests/test_local_storage_indexing.py::test_incremental_indexing_code_file expects AST symbols but finds zero. The same assertion was reproduced with unmodified application source from base 8491cc4. The full suite and a fresh source build are not claimed green.