feat(db): Postgres backend (khive-db-postgres) — SQL dialect + pgvector + pooling

[ohdearquant]

Part of #554 (multi-backend storage). ADR-009 already named khive-db-postgres as planned.

Why Postgres is the easy backend (and validates #555)

Postgres is SQL — it reuses the AST→SQL compiler path with a dialect fork, not a new query language. This is the concrete second consumer of #555's QueryCompiler strategy ("SQL dialects" plural in ADR-008). Dialect deltas vs SQLite:

• Param placeholders: $1, $2 vs SQLite ?.
• Recursive CTE: both WITH RECURSIVE, but type coercion + LIMIT semantics differ.
• Upsert: INSERT ... ON CONFLICT (mostly compatible, watch partial-index targets).
• Types: strict typing + jsonb (vs SQLite dynamic + json1) for entity properties.

Trait impls (khive-db-postgres crate, the 8 ADR-005 traits)

• SqlAccess/SqlReader/SqlWriter/SqlTransaction over a pooled tokio-postgres/sqlx connection.
• VectorStore → pgvector (HNSW/IVFFlat). Maps onto the VectorStoreCapabilities model (ADR-044): set supports_filter=true (pgvector + WHERE pushdown), declare VectorIndexKind::PgVector. This is option 2 of design(retrieval): vector storage & hybrid query under multi-backend graph DBs #557 for the SQL world — vectors co-located, native pre-filter.
• TextSearch → tsvector/tsquery or pg_trgm (FTS5 has no Postgres equivalent — needs its own impl + the FTS-escaping fix from bug(db): single-quote in search query crashes FTS5 ("syntax error near '") #549 re-validated for tsquery).
• Migrations: per-backend (migrations.rs is SQLite DDL today). Postgres gets its own migration set incl. RLS policies (see linked RLS issue).

Concurrency

Postgres MVCC removes SQLite's single-writer (WAL) bottleneck — required for a hosted multi-writer khive.

Acceptance

• khive-db-postgres crate implements all 8 traits; binary wires it via config.
• PostgresCompiler (feat(query): dialect-keyed query compiler — decouple AST→codegen from SQLite (Cypher/SurrealQL) #555) passes the GQL/SPARQL pattern parity suite.
• pgvector-backed VectorStore passes the vector search suite; hybrid RRF parity vs SQLite.
• smoke_test.py green against a Postgres backend.