…ase 1, SQLite)

Adds a new `zeph-db` Layer 0 crate that provides a compile-time database
abstraction over sqlx, currently supporting SQLite with PostgreSQL groundwork.

Architecture:
- `Dialect` trait with `Sqlite` and `Postgres` zero-sized marker types
- `DatabaseDriver` trait unifying pool construction, migrations, and
  transaction semantics; `SqliteDriver` impl in Phase 1
- `ActiveDriver` type alias gates the active backend at compile time
- `DbPool`/`DbRow`/`DbTransaction`/`ActiveDialect` derived from `ActiveDriver`
- `sql!()` macro for portable placeholder syntax (no-op on SQLite, rewrites
  `?` to `$N` on PostgreSQL)
- `DbConfig` with `write_pool_size = 1` default for SQLite single-writer safety
- `redact_url()` strips credentials from connection URLs in error messages
- `compile_error!` guard enforces mutual exclusivity of `sqlite`/`postgres`

Migrations:
- All 50 migrations moved from `zeph-memory/migrations/` to
  `zeph-db/migrations/sqlite/`
- Added `051_scheduler_jobs.sql` and `052_mcp_trust_scores.sql`, replacing
  inline DDL in `zeph-scheduler` and `zeph-mcp`

Consumer crate changes:
- `zeph-memory`: `sqlite/` module renamed to `store/`, `sqlite_vector_store.rs`
  renamed to `db_vector_store.rs`; `SqliteVectorStore` → `DbVectorStore`,
  `SqliteGraphStore` → `DbGraphStore`; 340+ query strings wrapped in `sql!()`
- `zeph-scheduler`: `JobStore::open()` now uses `DbConfig::connect()` for
  WAL/busy_timeout/foreign_keys; inline DDL removed
- `zeph-mcp`, `zeph-orchestration`, `zeph-index`: migrated to `zeph_db::DbPool`
  and `sql!()` macro
- `MemoryError::Sqlite` → `MemoryError::Sqlx` (backend-neutral name)
- `SchedulerError::Db` variant added

All existing tests pass (7117/7117).

…mer crates

- Replace datetime('now') with CURRENT_TIMESTAMP (standard SQL, both backends)
- Add Dialect::COLLATE_NOCASE ("COLLATE NOCASE" / "" for SQLite/Postgres)
- Remove Dialect::NOW (superseded by CURRENT_TIMESTAMP)
- Replace all INSERT OR IGNORE with ActiveDialect::INSERT_IGNORE + CONFLICT_NOTHING
  across db_vector_store, snapshot, mem_scenes, messages, acp_sessions, graph/store
- Replace COLLATE NOCASE literals with ActiveDialect::COLLATE_NOCASE in graph/store
- Migrate sanitize_fts_query callers to zeph_db::fts; remove local duplicate
- Fix activate_skill_version to use begin_write() instead of pool.begin()
- Remove dead MemoryError::Migration variant
- Add default = ["sqlite"] to zeph-db/Cargo.toml for standalone cargo check
- Fix DatabaseDriver trait: remove run_migrations/begin/begin_write (free fns only)
- Run rustfmt across all modified files

Tests: 7148/7148 passed