WJ-1139: add forum schema, prune references, and seed sequences

[Syxless]

Implements the forum schema (groups, categories, threads, locks, posts, post revisions) with site-level composite FKs, actor fields, soft-deletes, discussion-thread uniqueness, and indexes for ordering/activity/lookups.
Adds forum compatibility ID bumps in the seeder.
Updates TextService::prune() to keep forum post revision text hashes.

Tests: ran.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 2.88%. Comparing base (3e13a6e) to head (4195bd0).

Files with missing lines	Patch %	Lines
deepwell/src/database/seeder/mod.rs	0.00%	8 Missing ⚠️
deepwell/src/services/text.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #2612      +/-   ##
==========================================
- Coverage     2.88%   2.88%   -0.01%     
==========================================
  Files          156     156              
  Lines         7585    7599      +14     
==========================================
  Hits           219     219              
- Misses        7366    7380      +14     

Files with missing lines	Coverage Δ
deepwell/src/services/text.rs	0.00% <0.00%> (ø)
deepwell/src/database/seeder/mod.rs	0.00% <0.00%> (ø)

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