fix: blank dashboard — model filter fallback + empty-string model normalisation

[HaydenHaines]

Summary

This PR fixes two root causes of the blank dashboard that users have been reporting in issues #76, #88, #90, #93, #99, #106.

Root cause 1 — readURLModels returns empty set (JS)

When a user's model names in the DB don't contain 'opus', 'sonnet', or 'haiku', readURLModels filtered to zero billable models and returned an empty selectedModels set. Every row was then filtered out, producing a blank dashboard across all date ranges — including "All".

Fix: if no billable models are found, fall back to selecting ALL models.

// before
if (!param) return new Set(allModels.filter(m => isBillable(m)));

// after
if (!param) {
  const billable = allModels.filter(m => isBillable(m));
  return new Set(billable.length > 0 ? billable : allModels);
}

Root cause 2 — empty-string model not normalised to 'unknown' (SQL)

Turns can have model = '' (empty string, not NULL). COALESCE(model, 'unknown') only catches NULL — empty strings pass through as '', creating an invisible bucket that the model filter never selects. Fixed with COALESCE(NULLIF(model, ''), 'unknown') in all three query sites.

This PR also includes 7 previously accumulated fixes that were committed locally but not yet pushed upstream (session names display, concurrent HTTP server, pricing refactor, Cowork audit tracking, bookmarkable URL fix, range filter bounds fix, peak-hour highlighting removal).

Test plan

• python3 -m pytest --tb=short -q — all 130 tests pass
• New TestEmptyStringModel regression tests cover the COALESCE(NULLIF(...)) fix
• New test_read_url_models_falls_back_to_all_when_no_billable covers the JS fallback

Fixes #76, #106 (and partially resolves #88, #90, #93, #99)

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