docs: sync README and deployment guide with current codebase (#67)

- README: update packages/db to mention SQLite/LibSQL support alongside PostgreSQL
- README: add space.atbb.forum.board to the lexicon table (added by ATB-23)
- Deployment guide: fix stale plan doc path (moved to docs/plans/complete/)
- Deployment guide: update DATABASE_URL to document both PostgreSQL and SQLite formats
- Deployment guide: add SQLite overview paragraph to Section 4 Database Setup
- Deployment guide: update migration verification table list from 7 to 12 tables
- Deployment guide: bump version to 1.2, update date to 2026-02-26

README.md

···
 
 | Package | Description |
 |---------|-------------|
-| [`packages/db`](packages/db) | Drizzle ORM schema and connection factory for PostgreSQL |
+| [`packages/db`](packages/db) | Drizzle ORM schema and connection factory (PostgreSQL and SQLite/LibSQL) |
 | [`packages/lexicon`](packages/lexicon) | AT Proto lexicon schemas (YAML) and generated TypeScript types |
 
 ## Getting Started
···
 | `space.atbb.forum.forum` | Forum DID | Forum metadata (name, description) |
 | `space.atbb.forum.category` | Forum DID | Subforum / category definitions |
 | `space.atbb.forum.role` | Forum DID | Role definitions with permission tokens |
+| `space.atbb.forum.board` | Forum DID | Board definitions within a category |
 | `space.atbb.post` | User DID | Posts and replies (unified model) |
 | `space.atbb.membership` | User DID | User's membership in a forum |
 | `space.atbb.reaction` | User DID | Reactions on posts (like, upvote, etc.) |

docs/deployment-guide.md

···
 # atBB Deployment Guide
 
-**Version:** 1.1
-**Last Updated:** 2026-02-22
+**Version:** 1.2
+**Last Updated:** 2026-02-26
 **Audience:** System administrators deploying atBB to production
 
 > **Related Documentation:**
 > - [docs/trust-model.md](trust-model.md) — Trust model for self-hosted deployment: what the AppView controls, user data guarantees, and security implications
-> - [docs/plans/2026-02-11-deployment-infrastructure-design.md](plans/2026-02-11-deployment-infrastructure-design.md) — Architectural decisions and design rationale behind this deployment approach
+> - [docs/plans/complete/2026-02-11-deployment-infrastructure-design.md](plans/complete/2026-02-11-deployment-infrastructure-design.md) — Architectural decisions and design rationale behind this deployment approach
 
 ## Table of Contents
 
···
 
 | Variable | Description | Example |
 |----------|-------------|---------|
-| `DATABASE_URL` | PostgreSQL connection string | `postgresql://user:pass@host:5432/dbname?sslmode=require` |
+| `DATABASE_URL` | Database connection string (PostgreSQL or SQLite) | PostgreSQL: `postgresql://user:pass@host:5432/dbname?sslmode=require`; SQLite: `file:./atbb.db` |
 | `FORUM_DID` | Forum's AT Protocol DID | `did:plc:abcdef1234567890` |
 | `PDS_URL` | Personal Data Server URL | `https://bsky.social` |
 | `FORUM_HANDLE` | Forum's AT Protocol handle | `forum.example.com` |
···
 ---
 
 ## 4. Database Setup
+
+atBB supports two database backends:
+
+- **PostgreSQL** (recommended for production) — full-featured, suitable for multi-user/multi-server deployments
+- **SQLite/LibSQL** (lightweight alternative) — single-file database, ideal for small self-hosted forums. Use a `file:` prefix in `DATABASE_URL` (e.g. `file:./atbb.db`) and run the SQLite-specific migrations (`docker-compose.sqlite.yml` for Docker or set `database.type = "sqlite"` in the NixOS module).
+
+The rest of this section covers PostgreSQL provisioning. SQLite requires no separate server setup — just point `DATABASE_URL` at a file path.
 
 ### PostgreSQL Provisioning
 
···
 # List tables
 \dt
 
-# Expected output:
-#  Schema |       Name        | Type  |   Owner
-# --------+-------------------+-------+-----------
-#  public | categories        | table | atbb_user
-#  public | firehose_cursor   | table | atbb_user
-#  public | forums            | table | atbb_user
-#  public | memberships       | table | atbb_user
-#  public | mod_actions       | table | atbb_user
-#  public | posts             | table | atbb_user
-#  public | users             | table | atbb_user
+# Expected output (12 tables):
+#  Schema |         Name          | Type  |   Owner
+# --------+-----------------------+-------+-----------
+#  public | backfill_errors       | table | atbb_user
+#  public | backfill_progress     | table | atbb_user
+#  public | boards                | table | atbb_user
+#  public | categories            | table | atbb_user
+#  public | firehose_cursor       | table | atbb_user
+#  public | forums                | table | atbb_user
+#  public | memberships           | table | atbb_user
+#  public | mod_actions           | table | atbb_user
+#  public | posts                 | table | atbb_user
+#  public | role_permissions      | table | atbb_user
+#  public | roles                 | table | atbb_user
+#  public | users                 | table | atbb_user
 ```
 
 ### Migration Troubleshooting
···
 ### Required Environment Variables
 
 ```bash
+# PostgreSQL (production recommended)
 DATABASE_URL=postgresql://user:pass@host:5432/db?sslmode=require
+# SQLite (lightweight alternative)
+# DATABASE_URL=file:./atbb.db
 FORUM_DID=did:plc:xxxxxxxxxxxxx
 PDS_URL=https://bsky.social
 FORUM_HANDLE=forum.example.com