test: isolate parallel postgres test runs

[lan17]

Summary

• provision a unique Postgres schema per test run so concurrent worktrees can share one database safely
• scope SQLAlchemy engines and Alembic migrations to that schema in test contexts
• add coverage proving two stores can reuse one Postgres instance without leaking state across schemas

Why

Parallel backend test runs were sharing one database namespace and using global drop_all/create_all, so concurrent worktrees or subagents could destroy each other's tables mid-run.

Validation

• make check

[codecov]

Codecov Report

❌ Patch coverage is 91.93548% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.81%. Comparing base (298a5c5) to head (df60903).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/homesec/postgres_support.py	92.45%	4 Missing ⚠️
src/homesec/telemetry/db_log_handler.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
+ Coverage   83.73%   83.81%   +0.07%     
==========================================
  Files         110      111       +1     
  Lines        9814     9873      +59     
==========================================
+ Hits         8218     8275      +57     
- Misses       1596     1598       +2     

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