Set SQLite cache, mmap, and WAL synchronous pragmas

[SawyerHood]

Why

better-sqlite3 is synchronous and every query runs on the server event loop. The live database is ~1.9 GB with a 2 MiB page cache, no mmap, and synchronous = FULL (fsync on every commit). Cold page reads and per-commit fsync are the main source of sub-second stalls, not a missing index.

This is layer 2 of the event-loop stall stack. It depends on layer 1 (#1437) so the new pragmas can be confirmed from stall and slow-query logs.

What

createConnection now sets:

pragma	value	reason
cache_size	-262144 (256 MiB)	replace the 2 MiB default in front of a multi-GB file
synchronous	NORMAL	WAL-standard; no fsync per commit. Power loss can drop the last transactions; it cannot corrupt the file
mmap_size	1073741824 (1 GiB)	avoid a pread + copy on every page
busy_timeout	5000	wait for a lock instead of failing immediately
temp_store	MEMORY	keep temp sorts off disk

wal_autocheckpoint is left unchanged. A 38 MB WAL is a reader-starvation problem, not an autocheckpoint setting problem.

Test plan

• File-backed pragma test asserts the live values
• @bb/db typecheck + full test suite
• Server database-maintenance sweep tests (busy_timeout save/restore)

Prerequisite: #1437.

AGENT GENERATED: by Grok 4.6

[SawyerHood]

🚨 SLOP COP 🚨 · review

ELI5: This pull request gives SQLite more memory and reduces slow disk work.

I am the SlopCop. I am reviewing this pull request for security, code quality, architecture, performance, and product behavior.

[SawyerHood]

🚨 slopcop/review — temp_store = MEMORY removes the normal disk spill path.

The main connection serves broad thread searches. That query groups and sorts matches before its final limit. Large matches can keep temporary indexes in process memory. The new page cache can also use 256 MiB. Please keep the default file store, or add a measured memory limit.

[SawyerHood]

Removed temp_store = MEMORY on 64d5e58. The connection keeps the default file store so large thread-search sorts can spill to disk.

[SawyerHood]

🚨 slopcop/review — These exports add internal tuning policy to the package API.

No production code imports these constants. The test already imports them from connection.ts. Please remove the three root exports. This change will keep the public contract small.

[SawyerHood]

Removed the three root exports on 64d5e58. The test still imports the constants from connection.ts.

[SawyerHood]

🚨 SLOP COP 🚨 · review

ELI5: This change gives SQLite more memory and reduces disk waits. It can run faster, but memory-only scratch work can grow too much.

I found two items:

• Medium: temp_store = MEMORY removes the disk spill path for temporary indexes. Broad thread searches can create data-dependent temporary work. Keep the default file store, or add a measured memory limit.
• Low: The root package exports three tuning constants that no production code uses. Keep this policy private to the connection module.

The security review found no injection issue. All new pragma values use fixed constants. The NORMAL durability loss is explicit and matches SQLite behavior. A system failure can remove recent commits, but an application failure will not.

The architecture scan found one main database connection path. Plugin databases use a separate policy, so a shared setup helper would mix different contracts. The 5-second timeout already matches the better-sqlite3 default. Thus, this line does not add a new delay.

Validation passed:

• The @bb/db type check passed.
• All 380 @bb/db tests passed.
• All current GitHub checks passed.
• The dev server started from this commit.
• A browser opened the app and showed the empty thread view.

I posted two inline comments. I used a comment review only.