Enterprise: StoreBackend ABC + Turso/Supabase backends

[itdove]

Description

Optional centralized DB backends for teams that need real-time sharing without git sync delay.

StoreBackend ABC (extends #3)

Already defined in #3. This issue adds new implementations.

New Backends

Turso (libSQL)

• SQLite-compatible API — existing queries work with minimal changes
• FTS5 preserved
• Native float32 vectors for hybrid search
• Config: `store.type: "turso"`, `store.url`, `store.auth_token_env`

Supabase (Postgres + RLS)

• Row-level security at DB level — MCP server carries no ACL logic
• `pgvector` for vector search
• `pg_tsvector` for FTS
• Config: `store.type: "supabase"`, `store.url_env`, `store.anon_key_env`

AlloyDB / Cloud SQL

• Google Cloud managed Postgres
• `pgvector` support
• Pairs with Vertex AI embeddings

Data Migration

lore migrate --to turso|supabase
→ export all entries from local SQLite
→ create schema on remote backend
→ import entries
→ update config to point to new backend
→ local SQLite becomes offline fallback

Local SQLite Stays

Always available as offline fallback. When centralized DB is configured:

• Writes go to centralized DB
• Local SQLite syncs periodically for offline access
• If centralized DB unreachable → fall back to local

Acceptance

• Turso backend implements StoreBackend ABC
• Supabase backend implements StoreBackend ABC
• `lore migrate --to turso|supabase` migrates data
• Local SQLite works as offline fallback
• `lore init` backend selector works

Dependencies

• MVP complete
• Hybrid search: EmbeddingProvider ABC + sqlite-vec + RRF #14 (hybrid search — vector backends vary by store)