feat: restore filtered fulltext memory search

[loveRhythm1990]

What type of PR is this?

• feat (new feature)
• fix (bug fix)
• docs (documentation)
• style (formatting, no code change)
• refactor (code change that neither fixes a bug nor adds a feature)
• perf (performance improvement)
• test (adding or updating tests)
• chore (maintenance, tooling)
• build / ci (build or CI changes)

Which issue(s) this PR fixes

Refs #229 and restores the changes from #230.

What this PR does / why we need it

PR #230 was squash-merged as e46ba76, but PR #228 was merged six minutes later from the same parent commit. The second squash commit replaced the first one on main, so the full-text search implementation disappeared even though #230 remains marked as merged.

This PR reapplies #230 on top of the current main, which already contains #228, and resolves the shared-file overlap without duplicating the Python Memory.subject_id or Memory.extra_metadata fields.

• Restores POST /v1/memories/fulltext-search backed by MatrixOne full-text search.
• Restores exact metadata and fixed-field SQL pre-filters, deterministic score ordering, strict session filtering, and branch isolation.
• Restores sync and async Python SDK memories.fulltext_search() support.
• Keeps the structured POST /v1/memories/query API from feat: add structured memory queries #228 intact.
• Keeps full-text search out of MCP as originally intended.
• Restores the API reference, changelogs, SDK documentation, and regression coverage.

Validation:

• cargo check --manifest-path memoria/Cargo.toml -p memoria-storage -p memoria-service -p memoria-api
• cargo clippy --manifest-path memoria/Cargo.toml -p memoria-storage -p memoria-service -p memoria-api --lib -- -D warnings
• cargo test --manifest-path memoria/Cargo.toml -p memoria-storage metadata_and_fulltext_validation_are_enforced_at_storage_boundary
• cargo test --manifest-path memoria/Cargo.toml -p memoria-api --lib fulltext_memory_type_parser_rejects_blank_entries_but_allows_empty_arrays
• cargo test --manifest-path memoria/Cargo.toml -p memoria-api --test api_e2e test_api_fulltext -- --nocapture (3 passed)
• uv run --project sdk/python pytest sdk/python/tests/unit/test_memories.py sdk/python/tests/unit/test_memories_async.py -q (89 passed)
• uv run --project sdk/python ruff check sdk/python/src/memoria/resources/memories.py sdk/python/tests/unit/test_memories.py sdk/python/tests/unit/test_memories_async.py
• git diff --check