perf(db): expose measured SQLite storage profiles

[ohdearquant]

Body:

• Problem: SQLite cache, mmap, WAL checkpoint, and reader-pool tuning are hardcoded or implicit, making RAM and throughput tradeoffs difficult to reproduce.
• Proposed fix: add named storage profiles with explicit cache, mmap, WAL checkpoint, and reader-pool settings, plus benchmark guidance before changing defaults.
• Acceptance criteria: configuration exposes conservative profile defaults; tests verify settings are applied; docs list profile names and intended workload; no default change lands without benchmark data.
• Affected crates/ADRs: crates/khive-db/src/pool.rs:13, crates/khive-db/src/pool.rs:371, crates/khive-db/src/pool.rs:398.

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Source: khive-improvement-survey show — proposed backlog (propose-only synthesis). Dedup: NEW.