feat(core): dynamic custom frontmatter fields for CTF writeup metadata

- Add extra_fields JSONB column to documents table (migration 0003)
- CreateDocInput/UpdateDocInput: extraFields?: Record<string, string>
- parseFile: extracts non-standard frontmatter keys as extraFields
- stringifyFile: writes extraFields dynamically to YAML frontmatter
- toMeta: spreads extraFields from DB row into DocumentMeta
- DocForm: '+ Add field' UI for content creators to add any metadata
- API routes: splitPayload() separates standard vs custom fields
- Doc page: CustomFieldBadges dynamically renders any custom field
- Add db:migrate / db:push scripts + deploy workflow migration step
- Add CTF writeup template (content/writeups/ctf/template/)
- Add sample DEF CON writeup with event/challenge/category/difficulty/points
- Add @tailwindcss/typography for table rendering (prose classes)
- Add remark-gfm for GitHub Flavored Markdown table parsing

content/writeups/ctf/template/writeup-template.md

@@ -0,0 +1,76 @@
+---
+id: ctf-writeup-template
+title: "CTF Writeup Template — How to Structure a Challenge Writeup"
+type: writeup
+tags:
+  - ctf
+  - template
+  - methodology
+status: draft
+author: asep
+created_at: 2026-08-20
+updated_at: 2026-08-20
+---
+
+# CTF Writeup Template
+
+A consistent writeup structure helps reviewers and future-you reproduce the solve.
+Below is the recommended template. Delete this intro paragraph and fill each
+section.
+
+## Challenge Info
+
+| Field        | Value                |
+| ------------ | -------------------- |
+| **Event**    | `[Event Name]`       |
+| **Challenge**| `[Challenge Name]`   |
+| **Category** | `pwn` / `crypto` / `web` / `rev` / `forensics` / `misc` |
+| **Difficulty**| `easy` / `medium` / `hard` |
+| **Points**   | `[points at start]`  |
+| **Solves**   | `[number]`           |
+
+## Initial Recon
+
+What you see when you download the binary/file/URL. File type, basic
+inspection (`file`, `strings`, `checksec`, HTTP headers, etc.).
+
+## Approach
+
+Describe the high-level idea — what class of vulnerability or attack this
+belongs to.
+
+## Step-by-Step Solve
+
+Walk through each step with commands and output. Include:
+
+- Exact commands you ran
+- Key output (truncated if long, but enough to confirm)
+- Why each step works
+- Tool versions / versions if relevant
+
+### 1. Enumerate
+
+```
+$ command-here
+output-here
+```
+
+### 2. Find the vulnerability
+
+Explain what you found and how it maps to the approach.
+
+### 3. Craft the exploit
+
+Show the exploit script or payload, explain each part.
+
+## Flag
+
+```
+flag{...}
+```
+
+## Summary
+
+What you learned, what the intended solution was (if yours differed),
+and any pitfalls you hit along the way (e.g., "tool X version Y breaks
+on Z").