fix(db): interrupted or failed migrations are no longer marked applied

[fclairamb]

Implements the spec from #548 (included in this branch, moved to specs/done/2026/10/).

Stopping a container during its first start left a SQLite database that never started again (no such column: organization.default_escalation_policy_uid). bun marked each migration applied before running it, so a killed or failing migration was skipped on the next boot.

• Both backends: migrate.WithMarkAppliedOnSuccess(true). A failed or interrupted migration stays pending and runs again.
• SQLite first boot is atomic: the DB is built as solidping.db.init, checkpointed, then renamed. Leftover .init files are deleted at start.
• SQLite upgrades: VACUUM INTO solidping.db.pre-<migration>.bak before pending migrations, latest only, skipped when free disk is under 2x the DB size. A migration error names the backup.
• New migrations should be .tx.up.sql (documented in wiki/conventions/migrations.md). Existing files are not renamed.
• A startup no such column/table error is wrapped as "database looks half-migrated", with a runbook entry.

Tests: failed migration not recorded then retried (SQLite and embedded Postgres), interrupted/cancelled first boot leaves nothing, clean first boot leaves one file, backup cases.

Not done here: the manual docker check (docker rm -f 5 s into a fresh container, restart on the same volume, /api/mgmt/health is 200).

Closes #548

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 84.37500% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.07%. Comparing base (b74bf7c) to head (9db70b7).

Files with missing lines	Patch %	Lines
server/internal/db/sqlite/firstboot.go	75.38%	16 Missing ⚠️
server/internal/db/sqlite/sqlite.go	94.44%	2 Missing ⚠️
server/internal/db/sqlite/diskfree_unix.go	75.00%	1 Missing ⚠️
server/internal/db/sqlite/memtemplate.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #549      +/-   ##
==========================================
- Coverage   83.08%   83.07%   -0.01%     
==========================================
  Files        1197     1200       +3     
  Lines      123358   123466     +108     
==========================================
+ Hits       102488   102573      +85     
- Misses      20862    20885      +23     
  Partials        8        8              

Files with missing lines	Coverage Δ
server/internal/db/halfmigrated.go	100.00% <100.00%> (ø)
server/internal/db/postgres/postgres.go	66.48% <100.00%> (+0.05%)	⬆️
server/main.go	33.33% <100.00%> (+0.19%)	⬆️
server/internal/db/sqlite/diskfree_unix.go	75.00% <75.00%> (ø)
server/internal/db/sqlite/memtemplate.go	70.90% <66.66%> (-0.79%)	⬇️
server/internal/db/sqlite/sqlite.go	89.77% <94.44%> (+0.08%)	⬆️
server/internal/db/sqlite/firstboot.go	75.38% <75.38%> (ø)

... and 13 files with indirect coverage changes

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