perf: use prepared statements for frequently-called server DB queries (rebase of #1878)

[efiten]

Continues #1878 by @Joel-Claw, at their request. Both commits are theirs, authorship unchanged; I only rebased them onto master and resolved the conflict with #1909.

The conflict, and how it is resolved

Exactly the two places I named in the review on #1878: OpenDB and Close(). Both PRs rewrite them, and #1909 went first because it is the correctness fix.

OpenDB — kept #1909's pinned-connection detectSchema and added this PR's prepareStatements() after it:

derr := d.detectSchema(ctx, sc)
_ = sc.Close()
if derr != nil { conn.Close(); return nil, fmt.Errorf("schema detection failed: %w", derr) }
// Statements are prepared after schema detection so they can never be
// compiled against a schema mode that turned out to be wrong (#1901).
if err := d.prepareStatements(); err != nil { ... }

The ordering matters and is not arbitrary: preparing before detection would compile statements against a schema mode that #1909 exists to stop trusting.

Close() — kept this PR's statement closing and did not restore the WAL checkpoint. #1909 removed it deliberately: the handle is mode=ro, so PRAGMA wal_checkpoint(TRUNCATE) can only ever fail with "disk I/O error (778)" and was emitting a misleading storage-fault line on every shutdown. That reasoning survives; the statement closing is added in front of it.

Verification

• Both commits cherry-picked onto e5595ad9
• cmd/server builds
• Full cmd/server suite: ok, 0 failures (not just the targeted DB tests — after master briefly went red today from a two-PR interaction, a full local run seemed worth the two minutes)

Review points still open, none blocking

From my review on #1878, unchanged by the rebase:

1. Every SQL string now exists twice, once prepared and once as the stmtQueryRow fallback literal, with nothing keeping them in sync. The fallback is genuinely needed — twelve test helpers build &DB{conn: ...} directly and never call prepareStatements — but a constructor for those helpers would remove the duplication.
2. stmtCountObsLastHour and stmtCountObsLastDay are byte-identical SQL.
3. OpenDB now refuses to start rather than degrading when a Prepare fails. Contained today, since none of the 13 prepared queries touch a schema-conditional column, but the failure mode changed.

@Joel-Claw — your work, your credit. Ping me if you would rather take it back.

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https://claude.ai/code/session_01Wzwr3eXseyNM7Xj598djjE