Fix 502: extract materialized views into canonical files, detect migration collisions

[Asherlc]

Two concurrent PRs (#581, #583) created conflicting 0049_* migrations that both
DROP and recreate v_activity/activity_summary. The second migration set
statement_timeout=60s which caused CREATE MATERIALIZED VIEW to timeout, leaving
views dropped but not recreated — crash-looping the container and causing 502s.

Root cause: PostgreSQL materialized views can't be ALTERed to add columns, so
any change requires DROP+CREATE. When two migrations do this independently,
the second clobbers the first's changes. This is a systemic migration problem.

Fix:

• Extract view definitions into canonical files (drizzle/views/) that are the
  single source of truth. The migration runner recreates all views from these
  files after every migration run (similar to Flyway's repeatable migrations).
• Add duplicate migration prefix detection — the runner now refuses to run if
  two files share a numeric prefix (e.g., two 0049_* files), catching this
  class of conflict at startup instead of silently corrupting state.
• Merge both 0049 migrations' v_activity changes (timezone + source_external_ids)
  into the canonical v_activity.sql definition.
• Remove statement_timeout from migrations (was causing view creation to timeout).
• Strip view DROP/CREATE from individual migrations (now handled by canonical files).

https://claude.ai/code/session_01YWKdigbgtBfQYnkq4LwZHo

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.32%. Comparing base (8944c23) to head (8c74b3c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #587   +/-   ##
=======================================
  Coverage   96.31%   96.32%           
=======================================
  Files         258      258           
  Lines       38316    38370   +54     
  Branches     7620     7641   +21     
=======================================
+ Hits        36905    36959   +54     
  Misses       1411     1411           

Flag	Coverage Δ
integration	64.45% <45.16%> (-0.04%)	⬇️
unit	89.82% <100.00%> (+0.01%)	⬆️

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