feat(backend): add bootstrap package with startup validation and DB probe

[GRACENOBLE]

Summary

• Adds internal/bootstrap package with an App struct that owns all shared, validated dependencies (*sql.DB, Config, *slog.Logger)
• bootstrap.Run(ctx) validates required BLUEPRINT_DB_* env vars, opens the DB connection, and probes Postgres with up to 5 attempts using full-jitter exponential backoff (500ms base → 16s cap, 15s per-attempt deadline to handle Neon cold starts)
• Context is signal-aware — Ctrl-C during startup probes cancels immediately
• server.NewServer updated to accept *bootstrap.App instead of reading env vars itself; startup failures exit with code 1 and a message to stderr
• main.go follows the signal/bootstrap/serve pattern from the reference implementation

Test plan

• cd backend && make docker-run && make watch — server starts, bootstrap logs show config valid + postgres ready
• Remove a required env var (e.g. BLUEPRINT_DB_HOST) — process exits 1 with config error before attempting a connection
• Stop Postgres mid-probe — retry log lines appear, process exits after 5 attempts
• go vet ./... and go build ./... pass cleanly

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: da2bcd21-68e9-44d4-95a1-3ce8fa127ca9

📥 Commits

Reviewing files that changed from the base of the PR and between 5a033f6 and daa560b.

📒 Files selected for processing (3)

• backend/cmd/api/main.go
• backend/internal/bootstrap/bootstrap.go
• backend/internal/server/server.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch backend.bootstrap

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