[explorer/rest] fix: fail startup on Symbol database initialization errors and return 503 on runtime failures

[daoka]

What is the current behavior?

SymbolRestFacade creates its own database and catches initialization errors, allowing the application to start without a usable database. Block detail requests can then return 404 despite the database being unavailable.

This PR builds on #2580 and targets explorer/backend/rest-receipts.

What's the issue?

Database initialization failures are hidden behind a running application, and unavailable data can be confused with a missing resource. Database construction and cleanup also lack a clear application-level owner.

How have you changed the behavior?

• Create SymbolDatabase during application setup and inject it into SymbolRestFacade.
• Fail application creation when database initialization fails. A connected but unsynced database still permits startup and is reported through Health.
• Close the pool on setup failure, preserving the original exception when ordinary cleanup fails.
• Keep the pool available across requests and register application finalization cleanup. Add idempotent close and context manager support.
• Remove DB-absent facade states. Preserve 400 for invalid input and 404 for missing resources in readable data, while returning 503 for database read failures or unavailable synchronization state.

Startup compatibility changes: deployments that previously started without a database now fail during application creation. HTTP paths and response schemas are unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.25%. Comparing base (eb8ed00) to head (658b7f6).

Additional details and impacted files

@@                        Coverage Diff                         @@
##           explorer/backend/rest-receipts    #2581      +/-   ##
==================================================================
- Coverage                           97.43%   97.25%   -0.18%     
==================================================================
  Files                                 368      359       -9     
  Lines                               27148    25514    -1634     
  Branches                              251      251              
==================================================================
- Hits                                26451    24814    -1637     
- Misses                                690      693       +3     
  Partials                                7        7              

Flag	Coverage Δ
explorer-puller	?
explorer-rest	99.80% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
explorer/rest/rest/__init__.py	100.00% <100.00%> (ø)
explorer/rest/rest/db/SymbolDatabase.py	100.00% <100.00%> (ø)
explorer/rest/rest/facade/SymbolRestFacade.py	100.00% <100.00%> (ø)
explorer/rest/rest/routes/symbol.py	100.00% <100.00%> (ø)

... and 43 files with indirect coverage changes

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